The Ink4a tumor suppressor gene product, p19Arf, interacts with MDM2 and neutralizes MDM2's inhibition of p53

The INK4a gene encodes two distinct growth inhibitors--the cyclin-dependent kinase inhibitor p16Ink4a, which is a component of the Rb pathway, and the tumor suppressor p19Arf, which has been functionally linked to p53. Here we show that p19Arf potently suppresses oncogenic transformation in primary cells and that this function is abrogated when p53 is neutralized by viral oncoproteins and dominant-negative mutants but not by the p53 antagonist MDM2. This finding, coupled with the observations that p19Arf and MDM2 physically interact and that p19Rrf blocks MDM2-induced p53 degradation and transactivational silencing, suggests that p19Arf functions mechanistically to prevent MDM2's neutralization of p53. Together, our findings ascribe INK4a's potent tumor suppressor activity to the cooperative actions of its two protein products and their relation to the two central growth control pathways, Rb and p53.

Preclinical evidence of Alzheimer's disease in persons homozygous for the epsilon 4 allele for apolipoprotein E

BACKGROUND

Variants of the apolipoprotein E allele appear to account for most cases of late-onset Alzheimer's disease, and persons with two copies of the epsilon 4 allele appear to have an especially high risk of dementia. Positron-emission tomography (PET) has identified specific regions of the brain in which the rate of glucose metabolism declines progressively in patients with probable Alzheimer's disease. We used PET to investigate whether these same regions of the brain are affected in subjects homozygous for the epsilon 4 allele before the onset of cognitive impairment.

METHODS

Apolipoprotein E genotypes were established in 235 volunteers 50 to 65 years of age who reported a family history of probable Alzheimer's disease. Neurologic and psychiatric evaluations, a battery of neuropsychological tests, magnetic resonance imaging, and PET were performed in 11 epsilon 4 homozygotes and 22 controls without the epsilon 4 allele who were matched for sex, age, and level of education. An automated method was used to generate an aggregate surface-projection map that compared regional rates of glucose metabolism in the two groups.

RESULTS

The epsilon 4 homozygotes were cognitively normal. They had significantly reduced rates of glucose metabolism in the same posterior cingulate, parietal, temporal, and prefrontal regions as in previously studied patients with probable Alzheimer's disease. They also had reduced rates of glucose metabolism in additional prefrontal regions, which may be preferentially affected during normal aging.

CONCLUSIONS

In late middle age, cognitively normal subjects who are homozygous for the epsilon 4 allele for apolipoprotein E have reduced glucose metabolism in the same regions of the brain as in patients with probable Alzheimer's disease. These findings provide preclinical evidence that the presence of the epsilon 4 allele is a risk factor for Alzheimer's disease. PET may offer a relatively rapid way of testing future treatments to prevent Alzheimer's disease.

Monoamine oxidase: from genes to behavior

Cloning of MAO (monoamine oxidase) A and B has demonstrated unequivocally that these enzymes are made up of different polypeptides, and our understanding of MAO structure, regulation, and function has been significantly advanced by studies using their cDNA. MAO A and B genes are located on the X-chromosome (Xp11.23) and comprise 15 exons with identical intron-exon organization, which suggests that they are derived from the same ancestral gene. MAO A and B knock-out mice exhibit distinct differences in neurotransmitter metabolism and behavior. MAO A knock-out mice have elevated brain levels of serotonin, norephinephrine, and dopamine and manifest aggressive behavior similar to human males with a deletion of MAO A. In contrast, MAO B knock-out mice do not exhibit aggression and only levels of phenylethylamine are increased. Mice lacking MAO B are resistant to the Parkinsongenic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine. Both MAO A and B knock-out mice show increased reactivity to stress. These knock-out mice are valuable models for investigating the role of monoamines in psychoses and neurodegenerative and stress-related disorders.

Aggressive behavior and altered amounts of brain serotonin and norepinephrine in mice lacking MAOA

Deficiency in monoamine oxidase A (MAOA), an enzyme that degrades serotonin and norepinephrine, has recently been shown to be associated with aggressive behavior in men of a Dutch family. A line of transgenic mice was isolated in which transgene integration caused a deletion in the gene encoding MAOA, providing an animal model of MAOA deficiency. In pup brains, serotonin concentrations were increased up to ninefold, and serotonin-like immunoreactivity was present in catecholaminergic neurons. In pup and adult brains, norepinephrine concentrations were increased up to twofold, and cytoarchitectural changes were observed in the somatosensory cortex. Pup behavioral alterations, including trembling, difficulty in righting, and fearfulness were reversed by the serotonin synthesis inhibitor parachlorophenylalanine. Adults manifested a distinct behavioral syndrome, including enhanced aggression in males.

Connexin mutations in X-linked Charcot-Marie-Tooth disease

X-linked Charcot-Marie-Tooth disease (CMTX) is a form of hereditary neuropathy with demyelination. Recently, this disorder was mapped to chromosome Xq13.1. The gene for the gap junction protein connexin32 is located in the same chromosomal segment, which led to its consideration as a candidate gene for CMTX. With the use of Northern (RNA) blot and immunohistochemistry technique, it was found that connexin32 is normally expressed in myelinated peripheral nerve. Direct sequencing of the connexin32 gene showed seven different mutations in affected persons from eight CMTX families. These findings, a demonstration of inherited defects in a gap junction protein, suggest that connexin32 plays an important role in peripheral nerve.

Thermosensory activation of insular cortex

Temperature sensation is regarded as a submodality of touch, but evidence suggests involvement of insular cortex rather than parietal somatosensory cortices. Using positron emission tomography (PET), we found contralateral activity correlated with graded cooling stimuli only in the dorsal margin of the middle/posterior insula in humans. This corresponds to the thermoreceptive- and nociceptive-specific lamina I spinothalamocortical pathway in monkeys, and can be considered an enteroceptive area within limbic sensory cortex. Because lesions at this site can produce the post-stroke central pain syndrome, this finding supports the proposal that central pain results from loss of the normal inhibition of pain by cold. Notably, perceived thermal intensity was well correlated with activation in the right (ipsilateral) anterior insular and orbitofrontal cortices.

The maximal affinity of ligands

We explore the question of what are the best ligands for macromolecular targets. A survey of experimental data on a large number of the strongest-binding ligands indicates that the free energy of binding increases with the number of nonhydrogen atoms with an initial slope of approximately -1.5 kcal/mol (1 cal = 4.18 J) per atom. For ligands that contain more than 15 nonhydrogen atoms, the free energy of binding increases very little with relative molecular mass. This nonlinearity is largely ascribed to nonthermodynamic factors. An analysis of the dominant interactions suggests that van der Waals interactions and hydrophobic effects provide a reasonable basis for understanding binding affinities across the entire set of ligands. Interesting outliers that bind unusually strongly on a per atom basis include metal ions, covalently attached ligands, and a few well known complexes such as biotin-avidin.

Stages of progression in drug involvement from adolescence to adulthood: further evidence for the gateway theory

Sequential stages of involvement in alcohol and/or cigarettes, marijuana, other illicit drugs and medically prescribed psychoactive drugs from adolescence to adulthood are investigated in a longitudinal cohort that has been followed from ages 15 to 35. Alternative models of progression are tested for their goodness of fit. Four stages are identified: that of legal drugs, alcohol or cigarettes; marijuana; illicit drugs other than marijuana; and medically prescribed drugs. Whereas progression to illicit drugs among men is dependent upon prior use of alcohol, among women either cigarettes or alcohol is a sufficient condition for progression to marijuana. Age of onset and frequency of use at a lower stage of drug use are strong predictors of further progression.

The natural history of drug use from adolescence to the mid-thirties in a general population sample

OBJECTIVES

This study sought to describe patterns of initiation, persistence, and cessation in drug use in individuals from their late 20s to their mid-30s, within a broad perspective that spans 19 years from adolescence to adulthood.

METHODS

A fourth wave of personal interviews was conducted at ages 34-35 with a cohort of men and women (n = 1160) representative of adolescents formerly enrolled in New York State public secondary high schools. A school survey was administered at ages 15-16, and personal interviews with participants and school absentees were conducted at ages 24-25 and 28-29. Retrospective continuous histories of 12 drug classes were obtained at each follow-up.

RESULTS

There was no initiation into alcohol and cigarettes and hardly any initiation into illicit drugs after age 29, the age at which most use ceased. The largest proportion of new users was observed for prescribed psychoactives. Periods of highest use since adolescence based on relative and absolute criteria were delineated. Among daily users, the proportions of heavy users declined for alcohol and marijuana but not for cigarettes.

CONCLUSIONS

Cigarettes are the most persistent of any drug used. Drug-focused interventions must target adolescents and young adults.

Neuroanatomical correlates of externally and internally generated human emotion

OBJECTIVE

Positron emission tomography was used to investigate the neural substrates of normal human emotional and their dependence on the types of emotional stimulus.

METHOD

Twelve healthy female subjects underwent 12 measurements of regional brain activity following the intravenous bolus administration of [15O]H2O as they alternated between emotion-generating and control film and recall tasks. Automated image analysis techniques were used to characterize and compare the increases in regional brain activity associated with the emotional response to complex visual (film) and cognitive (recall) stimuli.

RESULTS

Film- and recall-generated emotion were each associated with significantly increased activity in the vicinity of the medial prefrontal cortex and thalamus, suggesting that these regions participate in aspects of emotion that do not depend on the nature of the emotional stimulus. Film-generated emotion was associated with significantly greater increases in activity bilaterally in the occipitotemporparietal cortex, lateral cerebellum, hypothalamus, and a region that includes the anterior temporal cortex, amygdala, and hippocampal formation, suggesting that these regions participate in the emotional response to certain exteroceptive sensory stimuli. Recall-generated sadness was associated with significantly greater increases in activity in the vicinity of the anterior insular cortex, suggesting that this region participates in the emotional response to potentially distressing cognitive or interoceptive sensory stimuli.

CONCLUSIONS

While this study should be considered preliminary, it identified brain regions that participate in externally and internally generated human emotion.

Neuroanatomical correlates of hunger and satiation in humans using positron emission tomography

The central role of the hypothalamus in the origination and/or processing of feeding-related stimuli may be modulated by the activity of other functional areas of the brain including the insular cortex (involved in enteroceptive monitoring) and the prefrontal cortex (involved in the inhibition of inappropriate response tendencies). Regional cerebral blood flow (rCBF), a marker of neuronal activity, was measured in 11 healthy, normal-weight men by using positron emission tomography in a state of hunger (after 36-h fast) and a state of satiation (after a liquid meal). Hunger was associated with significantly increased rCBF in the vicinity of the hypothalamus and insular cortex and in additional paralimbic and limbic areas (orbitofrontal cortex, anterior cingulate cortex, and parahippocampal and hippocampal formation), thalamus, caudate, precuneus, putamen, and cerebellum. Satiation was associated with increased rCBF in the vicinity of the ventromedial prefrontal cortex, dorsolateral prefrontal cortex, and inferior parietal lobule. Changes in plasma insulin concentrations in response to the meal were negatively correlated with changes in rCBF in the insular and orbitofrontal cortex. Changes in plasma free fatty acid concentrations in response to the meal were negatively correlated with changes in rCBF in the anterior cingulate and positively correlated with changes in rCBF in the dorsolateral prefrontal cortex. In conclusion, these findings raise the possibility that several regions of the brain participate in the regulation of hunger and satiation and that insulin and free fatty acids may be metabolic modulators of postprandial brain neuronal events. Although exploratory, the present study provides a foundation for investigating the human brain regions and cognitive operations that respond to nutritional stimuli.

Stereospecific alkane hydroxylation by non-heme iron catalysts: mechanistic evidence for an Fe(V)=O active species

High-valent iron-oxo species have frequently been invoked in the oxidation of hydrocarbons by both heme and non-heme enzymes. Although a formally Fe(V)=O species, that is, [(Por(*))Fe(IV)=O](+), has been widely accepted as the key oxidant in stereospecific alkane hydroxylation by heme systems, it is not established that such a high-valent state can be accessed by a non-heme ligand environment. Herein we report a systematic study on alkane oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, that is, [Fe(II)(TPA)(CH(3)CN)(2)](2+) (1, TPA = tris(2-pyridylmethyl)amine) and its alpha- and beta-substituted analogues. The reactivity patterns of this family of Fe(II)(TPA) catalysts can be modulated by the electronic and steric properties of the ligand environment, which affects the spin states of a common Fe(III)-OOH intermediate. Such an Fe(III)-peroxo species is high-spin when the TPA ligand has two or three alpha-substituents and is proposed to be directly responsible for the selective C-H bond cleavage of the alkane substrate. The thus-generated alkyl radicals, however, have relatively long lifetimes and are susceptible to radical epimerization and trapping by O(2). On the other hand, 1 and the beta-substituted Fe(II)(TPA) complexes catalyze stereospecific alkane hydroxylation by a mechanism involving both a low-spin Fe(III)-OOH intermediate and an Fe(V)=O species derived from O-O bond heterolysis. We propose that the heterolysis pathway is promoted by two factors: (a) the low-spin iron(III) center which weakens the O-O bond and (b) the binding of an adjacent water ligand that can hydrogen bond to the terminal oxygen of the hydroperoxo group and facilitate the departure of the hydroxide. Evidence for the Fe(V)=O species comes from isotope-labeling studies showing incorporation of (18)O from H(2)(18)O into the alcohol products. (18)O-incorporation occurs by H(2)(18)O binding to the low-spin Fe(III)-OOH intermediate, its conversion to a cis-H(18)O-Fe(V)=O species, and then oxo-hydroxo tautomerization. The relative contributions of the two pathways of this dual-oxidant mechanism are affected by both the electron donating ability of the TPA ligand and the strength of the C-H bond to be broken. These studies thus serve as a synthetic precedent for an Fe(V)=O species in the oxygen activation mechanisms postulated for non-heme iron enzymes such as methane monooxygenase and Rieske dioxygenases.

Developmental abnormalities and age-related neurodegeneration in a mouse model of Down syndrome

To study the pathogenesis of central nervous system abnormalities in Down syndrome (DS), we have analyzed a new genetic model of DS, the partial trisomy 16 (Ts65Dn) mouse. Ts65Dn mice have an extra copy of the distal aspect of mouse chromosome 16, a segment homologous to human chromosome 21 that contains much of the genetic material responsible for the DS phenotype. Ts65Dn mice show developmental delay during the postnatal period as well as abnormal behaviors in both young and adult animals that may be analogous to mental retardation. Though the Ts65Dn brain is normal on gross examination, there is age-related degeneration of septohippocampal cholinergic neurons and astrocytic hypertrophy, markers of the Alzheimer disease pathology that is present in elderly DS individuals. These findings suggest that Ts65Dn mice may be used to study certain developmental and degenerative abnormalities in the DS brain.

Declining brain activity in cognitively normal apolipoprotein E epsilon 4 heterozygotes: A foundation for using positron emission tomography to efficiently test treatments to prevent Alzheimer's disease

Cross-sectional positron emission tomography (PET) studies find that cognitively normal carriers of the apolipoprotein E (APOE) epsilon4 allele, a common Alzheimer's susceptibility gene, have abnormally low measurements of the cerebral metabolic rate for glucose (CMRgl) in the same regions as patients with Alzheimer's dementia. In this article, we characterize longitudinal CMRgl declines in cognitively normal epsilon4 heterozygotes, estimate the power of PET to test the efficacy of treatments to attenuate these declines in 2 years, and consider how this paradigm could be used to efficiently test the potential of candidate therapies for the prevention of Alzheimer's disease. We studied 10 cognitively normal epsilon4 heterozygotes and 15 epsilon4 noncarriers 50-63 years of age with a reported family history of Alzheimer's dementia before and after an interval of approximately 2 years. The epsilon4 heterozygotes had significant CMRgl declines in the vicinity of temporal, posterior cingulate, and prefrontal cortex, basal forebrain, parahippocampal gyrus, and thalamus, and these declines were significantly greater than those in the epsilon4 noncarriers. In testing candidate primary prevention therapies, we estimate that between 50 and 115 cognitively normal epsilon4 heterozygotes are needed per active and placebo treatment group to detect a 25% attenuation in these CMRgl declines with 80% power and P = 0.005 in 2 years. Assuming these CMRgl declines are related to the predisposition to Alzheimer's dementia, this study provides a paradigm for testing the potential of treatments to prevent the disorder without having to study thousands of research subjects or wait many years to determine whether or when treated individuals develop symptoms.

Tuning the activity of an enzyme for unusual environments: sequential random mutagenesis of subtilisin E for catalysis in dimethylformamide

Random mutagenesis has been used to engineer the protease subtilisin E to function in a highly nonnatural environment--high concentrations of a polar organic solvent. Sequential rounds of mutagenesis and screening have yielded a variant (PC3) that hydrolyzes a peptide substrate 256 times more efficiently than wild-type subtilisin in 60% dimethylformamide. PC3 subtilisin E and other variants containing different combinations of amino acid substitutions are effective catalysts for transesterification and peptide synthesis in dimethylformamide and other organic media. Starting with a variant containing four effective amino acid substitutions (D60N, D97G, Q103R, and N218S; where, for example, D60N represents Asp-60-->Asn), six additional mutations (G131D, E156G, N181S, S182G, S188P, and T255A) were generated during three sequential rounds of mutagenesis and screening. The 10 substitutions are clustered on one face of the enzyme, near the active site and substrate binding pocket, and all are located in loops that connect core secondary structure elements and exhibit considerable sequence variability in subtilisins from different sources. These variable surface loops are effective handles for "tuning" the activity of subtilisin. Seven of the 10 amino acid substitutions in PC3 are found in other natural subtilisins. Great variability is exhibited among naturally occurring sequences that code for similar three-dimensional structures--it is possible to make use of this sequence flexibility to engineer enzymes to exhibit features not previously developed (or required) for function in vivo.

NOTUNG: a program for dating gene duplications and optimizing gene family trees

Large scale gene duplication is a major force driving the evolution of genetic functional innovation. Whole genome duplications are widely believed to have played an important role in the evolution of the maize, yeast, and vertebrate genomes. The use of evolutionary trees to analyze the history of gene duplication and estimate duplication times provides a powerful tool for studying this process. Many studies in the molecular evolution literature have used this approach on small data sets, using analyses performed by hand. The rapid growth of genetic sequence data will soon allow similar studies on a genomic scale, but such studies will be limited unless the analysis can be automated. Even existing data sets admit alternative hypotheses that would be too tedious to consider without automation. In this paper, we describe a program called NOTUNG that facilitates large scale analysis, using both rooted and unrooted trees. When tested on trees analyzed in the literature, NOTUNG consistently yielded results that agree with the assessments in the original publications. Thus, NOTUNG provides a basic building block for inferring duplication dates from gene trees automatically and can also be used as an exploratory analysis tool for evaluating alternative hypotheses.

Persistently modified h-channels after complex febrile seizures convert the seizure-induced enhancement of inhibition to hyperexcitability

Febrile seizures are the most common type of developmental seizures, affecting up to 5% of children. Experimental complex febrile seizures involving the immature rat hippocampus led to a persistent lowering of seizure threshold despite an upregulation of inhibition. Here we provide a mechanistic resolution to this paradox by showing that, in the hippocampus of rats that had febrile seizures, the long-lasting enhancement of the widely expressed intrinsic membrane conductance Ih converts the potentiated synaptic inhibition to hyperexcitability in a frequency-dependent manner. The altered gain of this molecular inhibition-excitation converter reveals a new mechanism for controlling the balance of excitation-inhibition in the limbic system. In addition, here we show for the first time that h-channels are modified in a human neurological disease paradigm.

Occurrence and behavior of antibiotics in water and sediments from the Huangpu River, Shanghai, China

This study aims to determine the occurrence and behavior of five classes of 20 antibiotics in both water and sedimentary phases in the Huangpu River, which supplies drinking water to Shanghai City, China. Of the 20 antibiotics, sulfonamides showed the highest concentrations in water samples (34-859 ng L(-1)) while tetracyclines (average concentration at 18 μg kg(-1) dry weight) and macrolides (12 μg kg(-1) dry weight) dominated in sediment samples. The spatial distribution of antibiotics showed that the main polluting sources into the river were animal farming sites and the tributaries downstream of the Yuanxie River. In addition, linear relationships between logKoc and logKow, and between logKoc and log molecular weight, were demonstrated, suggesting the importance of contaminant properties in inter-phase behavior. The environmental risk assessment revealed that some antibiotics, in particular sulfamethoxazole could cause medium damage to daphnid in the aquatic ecosystem.

An amino-terminal domain of Mxi1 mediates anti-Myc oncogenic activity and interacts with a homolog of the yeast transcriptional repressor SIN3

Documented interactions among members of the Myc superfamily support a yin-yang model for the regulation of Myc-responsive genes in which transactivation-competent Myc-Max heterodimers are opposed by repressive Mxi1-Max or Mad-Max complexes. Analysis of mouse mxi1 has led to the identification of two mxi1 transcript forms possessing open reading frames that differ in their capacity to encode a short amino-terminal alpha-helical domain. The presence of this segment dramatically augments the suppressive potential of Mxi1 and allows for association with a mammalian protein that is structurally homologous to the yeast transcriptional repressor SIN3. These findings provide a mechanistic basis for the antagonistic actions of Mxi1 on Myc activity that appears to be mediated in part through the recruitment of a putative transcriptional repressor.

Influence of age and hemodynamics on myocardial blood flow and flow reserve

BACKGROUND

Aging is associated with changes of the systolic blood pressure that may increase cardiac work and myocardial blood flow at rest and reduce the myocardial flow reserve. This might be misinterpreted as age-related impairment of the coronary vasodilator capacity.

METHODS AND RESULTS

Myocardial blood flow was quantified at rest and after administration of intravenous dipyridamole in 40 healthy volunteers (12 women and 28 men) with 13N-ammonia and positron emission tomography. Eighteen of the normal subjects were less than and 22 were older than 50 years (31 +/- 9 versus 64 +/- 9 years). The resting rate-pressure product was lower in the younger than in the older subjects (6895 +/- 1070 versus 8634 +/- 1890; P < 0.01). Myocardial blood flow at rest averaged 0.76 +/- 0.17 mL.min-1.g-1 in the younger volunteers and 0.92 +/- 0.25 mL.min-1.g-1 in the older volunteers (P < 0.05). Hyperemic blood flows did not differ between younger and older subjects (3.0 +/- 0.8 versus 2.7 +/- 0.6 mL.min-1.g-1; P = NS); however, minimal coronary resistance was higher in the older subjects. Corrected for indexes of coronary driving pressure, hyperemic flow was lower in older than in younger normal subjects. The higher resting blood flows combined with similar hyperemic flows resulted in a lower myocardial flow reserve in the older than in the younger normal subjects (4.1 +/- 0.9 versus 3.0 +/- 0.70; P < 0.0001). The flow reserve was more closely correlated with resting than with hyperemic blood flows.

CONCLUSIONS

Aging does not alter significantly dipyridamole-induced hyperemic flows; although coronary vascular resistance after dipyridamole was somewhat increased in older subjects. The gradual decline of the myocardial blood flow reserve correlates with an age-related increase of baseline myocardial work and blood flow. These findings suggest that the reduced flow reserve with age is primarily due to increased cardiac work and blood flow at rest rather than to an abnormal vasodilator capacity.

The major protein import receptor of plastids is essential for chloroplast biogenesis

Light triggers the developmental programme in plants that leads to the production of photosynthetically active chloroplasts from non-photosynthetic proplastids. During this chloroplast biogenesis, the photosynthetic apparatus is rapidly assembled, mostly from nuclear-encoded imported proteins, which are synthesized in the cytosol as precursors with cleavable amino-terminal targeting sequences called transit sequences. Protein translocon complexes at the outer (Toc complex) and inner (Tic complex) envelope membranes recognize these transit sequences, leading to the precursors being imported. The Toc complex in the pea consists of three major components, Toc75, Toc34 and Toc159 (formerly termed Toc86). Toc159, which is an integral membrane GTPase, functions as a transit-sequence receptor. Here we show that Arabidopsis thaliana Toc159 (atToc159) is essential for the biogenesis of chloroplasts. In an Arabidopsis mutant (ppi2) that lacks atToc159, photosynthetic proteins that are normally abundant are transcriptionally repressed, and are found in much smaller amounts in the plastids, although ppi2 does not affect either the expression or the import of less abundant non-photosynthetic plastid proteins. These findings indicate that atToc159 is required for the quantitative import of photosynthetic proteins. Two proteins that are related to atToc159 (atToc120 and atToc132) probably help to maintain basal protein import in ppi2, and so constitute components of alternative, atToc159-independent import pathways.

Network dynamics and cell physiology

Complex assemblies of interacting proteins carry out most of the interesting jobs in a cell, such as metabolism, DNA synthesis, movement and information processing. These physiological properties play out as a subtle molecular dance, choreographed by underlying regulatory networks. To understand this dance, a new breed of theoretical molecular biologists reproduces these networks in computers and in the mathematical language of dynamical systems.