Molecular subclasses of high-grade glioma predict prognosis, delineate a pattern of disease progression, and resemble stages in neurogenesis

Previously undescribed prognostic subclasses of high-grade astrocytoma are identified and discovered to resemble stages in neurogenesis. One tumor class displaying neuronal lineage markers shows longer survival, while two tumor classes enriched for neural stem cell markers display equally short survival. Poor prognosis subclasses exhibit markers either of proliferation or of angiogenesis and mesenchyme. Upon recurrence, tumors frequently shift toward the mesenchymal subclass. Chromosomal locations of genes distinguishing tumor subclass parallel DNA copy number differences between subclasses. Functional relevance of tumor subtype molecular signatures is suggested by the ability of cell line signatures to predict neurosphere growth. A robust two-gene prognostic model utilizing PTEN and DLL3 expression suggests that Akt and Notch signaling are hallmarks of poor prognosis versus better prognosis gliomas, respectively.

Structure of the haemagglutinin membrane glycoprotein of influenza virus at 3 A resolution

The haemagglutinin glycoprotein of influenza virus is a trimer comprising two structurally distinct regions: a triple-stranded coiled-coil of alpha-helices extends 76 A from the membrane and a globular region of antiparallel beta-sheet, which contains the receptor binding site and the variable antigenic determinants, is positioned on top of this stem. Each subunit has an unusual loop-like topology, starting at the membrane, extending 135 A distally and folding back to enter the membrane.

Occludin: a novel integral membrane protein localizing at tight junctions

Recently, we found that ZO-1, a tight junction-associated protein, was concentrated in the so called isolated adherens junction fraction from the liver (Itoh, M., A. Nagafuchi, S. Yonemura, T. Kitani-Yasuda, Sa. Tsukita, and Sh. Tsukita. 1993. J. Cell Biol. 121:491-502). Using this fraction derived from chick liver as an antigen, we obtained three monoclonal antibodies specific for a approximately 65-kD protein in rats. This antigen was not extractable from plasma membranes without detergent, suggesting that it is an integral membrane protein. Immunofluorescence and immunoelectron microscopy with these mAbs showed that this approximately 65-kD membrane protein was exclusively localized at tight junctions of both epithelial and endothelial cells: at the electron microscopic level, the labels were detected directly over the points of membrane contact in tight junctions. To further clarify the nature and structure of this membrane protein, we cloned and sequenced its cDNA. We found that the cDNA encoded a 504-amino acid polypeptide with 55.9 kDa. A search of the data base identified no proteins with significant homology to this membrane protein. A most striking feature of its primary structure was revealed by a hydrophilicity plot: four putative membrane-spanning segments were included in the NH2-terminal half. This hydrophilicity plot was very similar to that of connexin, an integral membrane protein in gap junctions. These findings revealed that an integral membrane protein localizing at tight junctions is now identified, which we designated as "occludin."

Mitochondrial autophagy is an HIF-1-dependent adaptive metabolic response to hypoxia

Autophagy is a process by which cytoplasmic organelles can be catabolized either to remove defective structures or as a means of providing macromolecules for energy generation under conditions of nutrient starvation. In this study we demonstrate that mitochondrial autophagy is induced by hypoxia, that this process requires the hypoxia-dependent factor-1-dependent expression of BNIP3 and the constitutive expression of Beclin-1 and Atg5, and that in cells subjected to prolonged hypoxia, mitochondrial autophagy is an adaptive metabolic response which is necessary to prevent increased levels of reactive oxygen species and cell death.

Stat6 is required for mediating responses to IL-4 and for development of Th2 cells

Interleukin-4 (IL-4) stimulation of cells leads to the activation of multiple signaling pathways, one of which involves Stat6. We have generated Stat6-deficient mice by gene targeting in embryonic stem cells to determine the role of this transcription factor in mediating the biologic functions of IL-4. IL-4-induced increases in the cell surface expression of both MHC class II antigens and IL-4 receptor are completely abrogated, and lymphocytes from Stat6-deficient animals fail to proliferate in response to IL-4. Stat6-deficient B cells do not produce IgE following in vivo immunization with anti-IgD. In addition, Stat6-deficient T lymphocytes fail to differentiate into Th2 cells in response to either IL-4 or Il-13. These results demonstrate that, despite the existence of multiple signaling pathways activated by IL-4, Stat6 is essential for mediating responses to IL-4 lymphocytes.

Uncoupling protein-2: a novel gene linked to obesity and hyperinsulinemia

A mitochondrial protein called uncoupling protein (UCP1) plays an important role in generating heat and burning calories by creating a pathway that allows dissipation of the proton electrochemical gradient across the inner mitochondrial membrane in brown adipose tissue, without coupling to any other energy-consuming process. This pathway has been implicated in the regulation of body temperature, body composition and glucose metabolism. However, UCP1-containing brown adipose tissue is unlikely to be involved in weight regulation in adult large-size animals and humans living in a thermoneutral environment (one where an animal does not have to increase oxygen consumption or energy expenditure to lose or gain heat to maintain body temperature), as there is little brown adipose tissue present. We now report the discovery of a gene that codes for a novel uncoupling protein, designated UCP2, which has 59% amino-acid identity to UCP1, and describe properties consistent with a role in diabetes and obesity. In comparison with UCP1, UCP2 has a greater effect on mitochondrial membrane potential when expressed in yeast. Compared to UCP1, the gene is widely expressed in adult human tissues, including tissues rich in macrophages, and it is upregulated in white fat in response to fat feeding. Finally, UCP2 maps to regions of human chromosome 11 and mouse chromosome 7 that have been linked to hyperinsulinaemia and obesity. Our findings suggest that UCP2 has a unique role in energy balance, body weight regulation and thermoregulation and their responses to inflammatory stimuli.

Increased amyloid-beta42(43) in brains of mice expressing mutant presenilin 1

Mutations in the genes encoding amyloid-beta precursor protein (APP), presenilin 1 (PS1) and presenilin 2 (PS2) are known to cause early-onset, autosomal dominant Alzheimer's disease. Studies of plasma and fibroblasts from subjects with these mutations have established that they all alter amyloid beta-protein (beta APP) processing, which normally leads to the secretion of amyloid-beta protein (relative molecular mass 4,000; M(r) 4K; approximately 90% A beta1-40, approximately 10% A beta1-42(43)), so that the extracellular concentration of A beta42(43) is increased. This increase in A beta42(43) is believed to be the critical change that initiates Alzheimer's disease pathogenesis because A beta42(43) is deposited early and selectively in the senile plaques that are observed in the brains of patients with all forms of the disease. To establish that the presenilin mutations increase the amount of A beta42(43) in the brain and to test whether presenilin mutations act as true (gain of function) dominants, we have now constructed mice expressing wild-type and mutant presenilin genes. Analysis of these mice showed that overexpression of mutant, but not wild-type, PS1 selectively increases brain A beta42(43). These results indicate that the presenilin mutations probably cause Alzheimer's disease through a gain of deleterious function that increases the amount of A beta42(43) in the brain.

The genome of the kinetoplastid parasite, Leishmania major

Leishmania species cause a spectrum of human diseases in tropical and subtropical regions of the world. We have sequenced the 36 chromosomes of the 32.8-megabase haploid genome of Leishmania major (Friedlin strain) and predict 911 RNA genes, 39 pseudogenes, and 8272 protein-coding genes, of which 36% can be ascribed a putative function. These include genes involved in host-pathogen interactions, such as proteolytic enzymes, and extensive machinery for synthesis of complex surface glycoconjugates. The organization of protein-coding genes into long, strand-specific, polycistronic clusters and lack of general transcription factors in the L. major, Trypanosoma brucei, and Trypanosoma cruzi (Tritryp) genomes suggest that the mechanisms regulating RNA polymerase II-directed transcription are distinct from those operating in other eukaryotes, although the trypanosomatids appear capable of chromatin remodeling. Abundant RNA-binding proteins are encoded in the Tritryp genomes, consistent with active posttranscriptional regulation of gene expression.

Accelerated Alzheimer-type phenotype in transgenic mice carrying both mutant amyloid precursor protein and presenilin 1 transgenes

Genetic causes of Alzheimer's disease (AD) include mutations in the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2) genes. The mutant APP(K670N,M671L) transgenic line, Tg2576, shows markedly elevated amyloid beta-protein (A beta) levels at an early age and, by 9-12 months, develops extracellular AD-type A beta deposits in the cortex and hippocampus. Mutant PS1 transgenic mice do not show abnormal pathology, but do display subtly elevated levels of the highly amyloidogenic 42- or 43-amino acid peptide A beta42(43). Here we demonstrate that the doubly transgenic progeny from a cross between line Tg2576 and a mutant PS1M146L transgenic line develop large numbers of fibrillar A beta deposits in cerebral cortex and hippocampus far earlier than their singly transgenic Tg2576 littermates. In the period preceding overt A beta deposition, the doubly transgenic mice show a selective 41% increase in A beta42(43) in their brains. Thus, the development of AD-like pathology is substantially enhanced when a PS1 mutation, which causes a modest increase in A beta42(43), is introduced into Tg2576-derived mice. Remarkably, both doubly and singly transgenic mice showed reduced spontaneous alternation performance in a "Y" maze before substantial A beta deposition was apparent. This suggests that some aspects of the behavioral phenotype in these mice may be related to an event that precedes plaque formation.

Expression of a Delta homologue in prospective neurons in the chick

The product of the Delta gene, acting as ligand, and that of the Notch gene, acting as receptor, are key components in a lateral-inhibition signalling pathway that regulates the detailed patterning of many different tissues in Drosophila. During neurogenesis in particular, neural precursors, by expressing Delta, inhibit neighbouring Notch-expressing cells from becoming committed to a neural fate. Vertebrates are known to have several Notch genes, but their functions are unclear and their ligands hitherto unidentified. Here we identify and describe a chick Delta homologue, C-Delta-1. We show that C-Delta-1 is expressed in prospective neurons during neurogenesis, as new cells are being born and their fates decided. Our data from the chick, combined with parallel evidence from Xenopus, suggest that both the Delta/Notch signalling mechanism and its role in neurogenesis have been conserved in vertebrates.

Claudins and epithelial paracellular transport

Tight junctions form continuous intercellular contacts controlling solute movement through the paracellular pathway across epithelia. Paracellular barriers vary among epithelia in electrical resistance and behave as if they are lined with pores that have charge and size selectivity. Recent evidence shows that claudins, a large family (at least 24 members) of intercellular adhesion molecules, form the seal and its variable pore-like properties. This evidence comes from the study of claudins expressed in cultured epithelial cell models, genetically altered mice, and human mutants. We review information on the structure, function, and transcriptional and posttranslational regulation of the claudin family as well as of their evolutionarily distant relatives called the PMP22/EMP/MP20/claudin, or pfam00822, superfamily.

Identification of nitric oxide synthase as a protective locus against tuberculosis

Mutagenesis of the host immune system has helped identify response pathways necessary to combat tuberculosis. Several such pathways may function as activators of a common protective gene: inducible nitric oxide synthase (NOS2). Here we provide direct evidence for this gene controlling primary Mycobacterium tuberculosis infection using mice homozygous for a disrupted NOS2 allele. NOS2(-/-) mice proved highly susceptible, resembling wild-type littermates immunosuppressed by high-dose glucocorticoids, and allowed Mycobacterium tuberculosis to replicate faster in the lungs than reported for other gene-deficient hosts. Susceptibility appeared to be independent of the only known naturally inherited antimicrobial locus, NRAMP1. Progression of chronic tuberculosis in wild-type mice was accelerated by specifically inhibiting NOS2 via administration of N6-(1-iminoethyl)-L-lysine. Together these findings identify NOS2 as a critical host gene for tuberculostasis.

The combined effects of tryptophan starvation and tryptophan catabolites down-regulate T cell receptor zeta-chain and induce a regulatory phenotype in naive T cells

Tryptophan catabolism is a tolerogenic effector system in regulatory T cell function, yet the general mechanisms whereby tryptophan catabolism affects T cell responses remain unclear. We provide evidence that the short-term, combined effects of tryptophan deprivation and tryptophan catabolites result in GCN2 kinase-dependent down-regulation of the TCR zeta-chain in murine CD8+ T cells. TCR zeta down-regulation can be demonstrated in vivo and is associated with an impaired cytotoxic effector function in vitro. The longer-term effects of tryptophan catabolism include the emergence of a regulatory phenotype in naive CD4+CD25- T cells via TGF-beta induction of the forkhead transcription factor Foxp3. Such converted cells appear to be CD25+, CD69-, CD45RBlow, CD62L+, CTLA-4+, BTLAlow and GITR+, and are capable of effective control of diabetogenic T cells when transferred in vivo. Thus, both tryptophan starvation and tryptophan catabolites contribute to establishing a regulatory environment affecting CD8+ as well as CD4+ T cell function, and not only is tryptophan catabolism an effector mechanism of tolerance, but it also results in GCN2-dependent generation of autoimmune-preventive regulatory T cells.

Blockade of Dll4 inhibits tumour growth by promoting non-productive angiogenesis

Tumour growth requires accompanying expansion of the host vasculature, with tumour progression often correlated with vascular density. Vascular endothelial growth factor (VEGF) is the best-characterized inducer of tumour angiogenesis. We report that VEGF dynamically regulates tumour endothelial expression of Delta-like ligand 4 (Dll4), which was previously shown to be absolutely required for normal embryonic vascular development. To define Dll4 function in tumour angiogenesis, we manipulated this pathway in murine tumour models using several approaches. Here we show that blockade resulted in markedly increased tumour vascularity, associated with enhanced angiogenic sprouting and branching. Paradoxically, this increased vascularity was non-productive-as shown by poor perfusion and increased hypoxia, and most importantly, by decreased tumour growth-even for tumours resistant to anti-VEGF therapy. Thus, VEGF-induced Dll4 acts as a negative regulator of tumour angiogenesis; its blockade results in a striking uncoupling of tumour growth from vessel density, presenting a novel therapeutic approach even for tumours resistant to anti-VEGF therapies.

The ER chaperone and signaling regulator GRP78/BiP as a monitor of endoplasmic reticulum stress

The multiple implications of ER stress and the unfolded protein response in health and disease highlight the importance of identifying convenient monitoring systems for its onset under various experimental or physiological settings. A large volume of studies establish that induction of GRP78 is a marker for ER stress. GRP78, also referred to as BiP, is a central regulator for ER stress due to its role as a major ER chaperone with anti-apoptotic properties as well as its ability to control the activation of transmembrane ER stress sensors (IRE1, PERK, and ATF6) through a binding-release mechanism. In the following report, we present several methods to measure GRP78 induction. This can be achieved by measuring the Grp78 promoter activity or by measuring the level of Grp78 transcripts or GRP78 protein. These techniques can be applied to tissue culture cells as well as tissues and organs.

A novel form of TNF/cachectin is a cell surface cytotoxic transmembrane protein: ramifications for the complex physiology of TNF

Tumor necrosis factor (TNF) is a monocyte-derived cytotoxin that has been implicated in tumor regression, septic shock, and cachexia. The mechanism by which TNF induces these different disease states is unclear. We have identified and characterized a novel, rapidly inducible cell surface cytotoxic integral transmembrane form of TNF. The existence and behavior of this novel form of TNF may explain the complex physiology of this molecule. We suggest that activated monocytes synthesize transmembrane TNF at the site of inflammation and kill their targets by either cell-to-cell contact or local release of the TNF secretory component. In contrast, septic shock and cachexia may result from either acute or chronic systemic activation of monocytes, resulting in the widespread release of TNF secretory component into the circulation of the affected individual. We further suggest that cell borne cytokines and cytotoxins may be the primary mediators of directed inflammatory responses.

Accelerated amyloid deposition in the brains of transgenic mice coexpressing mutant presenilin 1 and amyloid precursor proteins

Missense mutations in two related genes, termed presenilin 1 (PS1) and presenilin 2 (PS2), cause dementia in a subset of early-onset familial Alzheimer's disease (FAD) pedigrees. In a variety of experimental in vitro and in vivo settings, FAD-linked presenilin variants influence the processing of the amyloid precursor protein (APP), leading to elevated levels of the highly fibrillogenic Abeta1-42 peptides that are preferentially deposited in the brains of Alzheimer Disease (AD) patients. In this report, we demonstrate that transgenic animals that coexpress a FAD-linked human PS1 variant (A246E) and a chimeric mouse/human APP harboring mutations linked to Swedish FAD kindreds (APP swe) develop numerous amyloid deposits much earlier than age-matched mice expressing APP swe and wild-type Hu PS1 or APP swe alone. These results provide evidence for the view that one pathogenic mechanism by which FAD-linked mutant PS1 causes AD is to accelerate the rate of beta-amyloid deposition in brain.

CFTR expression and chloride secretion in polarized immortal human bronchial epithelial cells

A major limitation in the study of vectorial ion transport, secretion, and differentiated function in the human airway epithelium has been the lack of suitable cell culture systems. Progress in this direction has been made through the transformation of primary cultured epithelial cells. However, these transformants tend to lose differentiated properties with increasing serial passage, particularly following crisis. The successful establishment of a postcrisis SV40 large T-antigen transformed epithelial cell line derived from human bronchial epithelium is described. This cell line, 16HBE14o-, retains differentiated epithelial morphology and functions. Cell cultures show the presence of tight junctions and cilia, and monolayers generate transepithelial resistance, as measured in Ussing chambers, and retain beta-adrenergic stimulation of cAMP-dependent chloride ion transport, measured either by 36Cl- efflux or as short-circuit current in Ussing chambers. The cells also increase chloride transport in response to bradykinin or calcium ionophore. In addition, 16HBE14o- cells express levels of both the cystic fibrosis transmembrane conductance regulator (CFTR) mRNA and protein readily detectable by Northern and Western hybridization analysis, respectively. These cells provide a valuable resource for studying the modulation of CFTR and its role in regulation of chloride ion transport in human airway epithelium as well as other aspects of human airway cell biology.

Membrane asymmetry

The components of biological membranes are asymmetrically distributed between the membrane surfaces. Proteins are absolutely asymmetrical in that every copy of a polypeptide chain has the same orientation in the membrane, and lipids are nonabsolutely asymmetrical in that almost every type of lipid is present on both sides of the bilayer, but in different and highly variable amounts. Asymmetry is maintained by lack of transmembrane diffusion. Two types of membrane proteins, called ectoproteins and endoproteins, are distinguished. Biosynthetic pathways for both types of proteins and for membrane lipids are inferred from their topography and distribution in the formed cells. Note added in proof. A cell-free system has now been developed which permits the mechanisms of membrane protein assembly to be studied (108). The membrane glycoprotein of vesicular stomatitis virus has been synthesized by wheat germ ribosomes in the presence of rough endoplasmic reticulum from pancreas. The resulting polypeptide is incorporated into the membrane, spans the lipid bilayer asymmetrically, and is glycosylated (108). The amino terminal portion of this transmembrane protein is found inside the endoplasmic reticulum vesicle, while the carboxyl terminal portion is exposed on the outer surface of the vesicle. Furthermore, addition of the glycoprotein to membranes after protein synthesis does not result in incorporation of the protein into the membrane in the manner described above (108). Consequently, protein synthesis and incorporation into the membrane must be closely coupled. Indeed, using techniques to synchronize the growth of nascent polypeptides, it has been shown (109) that no more than one-fourth of the glycoprotein chain can be made in the absence of membranes and still cross the lipid bilayer when chains are subsequently completed in the presence of membranes. These findings demonstrate directly that the extracytoplasmic portion of an ectoprotein can cross the membrane only during biosynthesis, and not after.

VIP21/caveolin is a cholesterol-binding protein

VIP21/caveolin is localized to both caveolae and apical transport vesicles and presumably cycles between the cell surface and the Golgi complex. We have studied the lipid interactions of this protein by reconstituting Escherichia coli-expressed VIP21/caveolin into liposomes. Surprisingly, the protein reconstituted only with cholesterol-containing lipid mixtures. We demonstrated that the protein binds at least 1 mol of cholesterol per mole of protein and that this binding promotes formation of protein oligomers. These findings suggest that VIP21/caveolin, through its cholesterol-binding properties, serves a specific function in microdomain formation during membrane trafficking.

Angiotensin II induces hypertrophy, not hyperplasia, of cultured rat aortic smooth muscle cells

We have explored the hypothesis that contractile agonists are important regulators of smooth muscle cell growth by examining the effects of one potent contractile agonist, angiotensin II (AII), on both cell proliferation and cellular hypertrophy. AII neither stimulated proliferation of cells made quiescent in a defined serum-free media nor augmented cell proliferation induced by serum or platelet-derived growth factor. However, AII did induce cellular hypertrophy of postconfluent quiescent cultures following 4 days of treatment, increasing smooth muscle cell protein content by 20% as compared with vehicle-treated controls. AII-induced hypertrophy was maximal at 1 microM, had an ED50 of 5 nM, and was blocked by the specific AII receptor antagonist Sar1,Ile8 AII. The cellular hypertrophy was due to an increase in protein synthesis, which was elevated within 6-9 hours following AII treatment, while no changes in protein degradation were apparent. AII was even more effective in inducing hypertrophy of subconfluent cultures, causing a 38% increase in protein content after 4 days of treatment (1 microM) and showing a maximal response at concentrations as low as 0.1 nM. Interestingly, in subconfluent cultures, AII treatment (1 microM, 4 days) was associated with a 50% increase in the fraction of cells with 4C DNA content with the virtual absence of cells in S-phase of the cell cycle, consistent with either arrest of cells in the G2 phase of the cell cycle or development of tetraploidy.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional dissociation of paracellular permeability and transepithelial electrical resistance and disruption of the apical-basolateral intramembrane diffusion barrier by expression of a mutant tight junction membrane protein

Tight junctions, the most apical of the intercellular junctions that connect individual cells in a epithelial sheet, are thought to form a seal that restricts paracellular and intramembrane diffusion. To analyze the functioning of tight junctions, we generated stable MDCK strain 2 cell lines expressing either full-length or COOH-terminally truncated chicken occludin, the only known transmembrane component of tight junctions. Confocal immunofluorescence and immunoelectron microscopy demonstrated that mutant occludin was incorporated into tight junctions but, in contrast to full-length chicken occludin, exhibited a discontinuous junctional staining pattern and also disrupted the continuous junctional ring formed by endogenous occludin. This rearrangement of occludin was not paralleled by apparent changes in the junctional morphology as seen by thin section electron microscopy nor apparent discontinuities of the junctional strands observed by freeze-fracture. Nevertheless, expression of both wild-type and mutant occludin induced increased transepithelial electrical resistance (TER). In contrast to TER, particularly the expression of COOH-terminally truncated occludin led to a severalfold increase in paracellular flux of small molecular weight tracers. Since the selectivity for size or different types of cations was unchanged, expression of wild-type and mutant occludin appears to have activated an existing mechanism that allows selective paracellular flux in the presence of electrically sealed tight junctions. Occludin is also involved in the formation of the apical/basolateral intramembrane diffusion barrier, since expression of the COOH-terminally truncated occludin was found to render MDCK cells incapable of maintaining a fluorescent lipid in a specifically labeled cell surface domain.

Loss of FBP1 by Snail-mediated repression provides metabolic advantages in basal-like breast cancer

The epithelial-mesenchymal transition (EMT) enhances cancer invasiveness and confers tumor cells with cancer stem cell (CSC)-like characteristics. We show that the Snail-G9a-Dnmt1 complex, which is critical for E-cadherin promoter silencing, is also required for the promoter methylation of fructose-1,6-biphosphatase (FBP1) in basal-like breast cancer (BLBC). Loss of FBP1 induces glycolysis and results in increased glucose uptake, macromolecule biosynthesis, formation of tetrameric PKM2, and maintenance of ATP production under hypoxia. Loss of FBP1 also inhibits oxygen consumption and reactive oxygen species production by suppressing mitochondrial complex I activity; this metabolic reprogramming results in an increased CSC-like property and tumorigenicity by enhancing the interaction of β-catenin with T-cell factor. Our study indicates that the loss of FBP1 is a critical oncogenic event in EMT and BLBC.

Multiple mechanisms of protein insertion into and across membranes

Protein localization in cells is initiated by the binding of characteristic leader (signal) peptides to specific receptors on the membranes of mitochondria or endoplasmic reticulum or, in bacteria, to the plasma membrane. There are differences in the timing of protein synthesis and translocation into or across the bilayer and in the requirement for a transmembrane electrochemical potential. Comparisons of protein localization in these different membranes suggest underlying common mechanisms.

Probing gene expression in live cells, one protein molecule at a time

We directly observed real-time production of single protein molecules in individual Escherichia coli cells. A fusion protein of a fast-maturing yellow fluorescent protein (YFP) and a membrane-targeting peptide was expressed under a repressed condition. The membrane-localized YFP can be detected with single-molecule sensitivity. We found that the protein molecules are produced in bursts, with each burst originating from a stochastically transcribed single messenger RNA molecule, and that protein copy numbers in the bursts follow a geometric distribution. The quantitative study of low-level gene expression demonstrates the potential of single-molecule experiments in elucidating the workings of fundamental biological processes in living cells.