Microsomal carbonyl reductase responsible for reduction of 4-phenyl-3-buten-2-one in rats

trans-4-Phenyl-3-buten-2-one (PBO), a flavoring additive, was transformed to the carbonyl-reduced product, trans-4-phenyl-3-buten-2-ol (PBOL) by rat liver microsomes, but not by liver cytosol, in the presence of NADH or NADPH. PBOL formed was identified by comparison with an authentic sample. The reductase activity was not inhibited by quercitrin, an inhibitor of cytosolic carbonyl reductase. The carbonyl reduction product of PBO by liver microsomes was identified as the R-enantiomer of PBOL by HPLC analysis. Rat blood also exhibited the carbonyl reductase activity in the presence of NADH or NADPH, but to a lesser extent.

A new analytical method for the detection of anti-tumor promoters using flow cytometry

Flow cytometry analysis was applied to the measurement of Epstein-Barr virus (EBV) induction which is used as a short term assay for anti-tumor promoters. The data obtained by measurement with flow cytometry were parallel with those of the fluorescence microscopic method. Flow cytometry is rapid, quantitative and should be applicable for the EBV activating test.

CEPU-1, an immunoglobulin superfamily molecule, has cell adhesion activity and shows dynamic expression patterns in chick embryonic spinal cord

In an attempt to isolate novel molecules involved in motoneuron differentiation and target muscle innervation during embryogenesis, we performed mRNA differential display analysis by comparing cDNAs of motoneurons purified by immunopanning from different portions along the rostro-caudal axis of chick embryonic spinal cord, and cloned an immunoglobulin superfamily protein named C30. By sequence comparison, C30 was shown to be an alternatively spliced isoform of CEPU-1, which was formerly reported as a member of the immunoglobulin superfamily specifically expressed in cerebellar Purkinje cells (Spaltmann and Brummendorf, 1996, J. Neurosci. 16, 1770-1779). We analyzed the expression pattern of CEPU-1 both at the mRNA and protein levels in the spinal cord of the chick embryo. Until stage 23, CEPU-1 was expressed faintly in the ventral part of the neural tube but gradually it became localized to a specific group of cells. In the motor column, CEPU-1 was expressed transiently in many columnar layers. A C30-transfected cell line showed Ca(2+)-independent cell-cell binding activity. These results suggest a role for CEPU-1 in specific axon guidance and/or fasciculation of motoneurons during development.

Cases of cutaneous and nasal CD56 (NCAM)-positive lymphoma in Japan have differences in immunohistology, genotype, and etiology

CD56 (NCAM)-positive lymphoma frequently involves the skin and nasal area. This study shows it is likely that the clinicopathologic features of this lymphoma are distinctive to each of the primarily involved sites. Sixteen cutaneous and 11 nasal cases of CD56-positive lymphoma were examined. In 10 cutaneous cases, the lesions consisted of pleomorphic large-cell lymphoma that expressed CD3epsilon (CD3), CD2 (LFA2), CD4, CD122 (IL-2B receptor), TIA1, perforin, and granzyme B and displayed angiocentric/angiodestructive features. Lobular panniculitis was found in 8 of these cases, and 6 cases showed other organ involvement. The remaining 6 cutaneous cases consisted mostly of CD3epsilon- and CD4-positive, CD2-, CD122-, and TIA1-negative large blastic lymphoma, having less angiodestruction and panniculitis. Bone marrow invasion and leukemic changes were found in 4 of these cases during the clinical course. All 11 nasal cases showed pleomorphic small and medium-size lymphoma cells with angiodestructive features and were positive for CD3epsilon, CD2, CD122, TIA1, perforin, and granzyme B. CD4-positive lymphoma was found in 4 of these cases. Only 3 nasal cases showed other organ involvement. Genotypically, 2 of the 4 cases examined in the first cutaneous group, 3 of the 4 cases examined in the second cutaneous group, and only 1 of the 11 nasal cases showed rearrangement of the TcRCbeta gene by the Southern blot method. Only 2 cutaneous cases with panniculitis and all 11 nasal cases showed a positive nuclear signal for EBV-encoded RNA (EBERs) by in situ hybridization. Thus, two types of cutaneous CD56-positive lymphoma were found, each having a unique cell characteristic, genotype, and EBV infection pattern that differed from that of nasal-type lymphoma.

New type of febrifugine analogues, bearing a quinolizidine moiety, show potent antimalarial activity against Plasmodium malaria parasite

Febrifugine (1) and isofebrifugine (2), isolated from the roots of Dichroa febrifuga Lour. (Chinese name: Cháng Shan), are active principles against malaria. Adducts of 1 and 2 with acetone, Df-1 (3) and Df-2 (4), respectively, were obtained using silica gel and acetone. They showed high activity against P. falciparum malaria in vitro. Compound 3 was found to be equally effective against P. berghei in vivo as the clinically used drug chloroquine, whereas 4 showed only 1/24 of the activity of 3. Metabolism studies of these compounds revealed that compound 4 is readily metabolized in mouse liver. Accordingly, the dose of 4 must be higher than that of 3 to attain blood levels sufficient for a favorable therapeutic effect.

Reductive metabolism In vivo of trans-4-phenyl-3-buten-2-one in rats and dogs

The reductive metabolism in vivo of a flavoring additive, trans-4-phenyl-3-buten-2-one (PBO; trans-methyl styryl ketone) was investigated in rats and dogs. In both species, the double bond-reduced product, 4-phenyl-2-butanone (PBA), was detected by HPLC as the predominant species in blood after i.v. administration of PBO. PBA detected in rat blood was identified by comparison to the authentic sample. In contrast, the carbonyl-reduced product, trans-4-phenyl-3-buten-2-ol (PBOL) was also detected as a minor metabolite of PBO in both species. The area under the curve of PBOL in rat blood was only 3% of that of PBA. PBO was mutagenic in the Ames test using Salmonella typhimurium TA 100 when S-9 mix was added, but PBA and PBOL were not. It appears that PBO is mainly metabolized to PBA in vivo in rats and dogs as a detoxification pathway.

Primary central nervous system T-cell lymphoma. Case report

A 46-year-old male presented with a rare primary non-Hodgkin's lymphoma of the central nervous system of T-cell lineage, localized primarily in the right parietal region. There was no evidence of acquired immunodeficiency syndrome. Biopsy of the tumor allowed immunohistochemical confirmation of the diagnosis. Irradiation and chemotherapy were given, and the patient has remained well for 24 months. The clinical manifestations, management, and outcome of T-cell lymphoma are very similar to those of B-cell lymphoma.

Treatment of sudden sensorineural hearing loss with a continuous epidural block

The efficacy of a continuous epidural block was evaluated in the treatment of 20 patients with sudden sensorineural hearing loss (SNHL). Ten patients were treated with continuous cervicothoracic epidural block (group A) and the other 10 were treated with stellate ganglion block (group B). No significant difference in factors affecting prognosis was noted between the groups. In the epidural block group 70% achieved substantial hearing improvement, while this occurred in only 30% of patients undergoing stellate ganglion block. These findings suggest that continuous epidural block can be effective in the treatment of sudden SNHL.

Current treatment strategies and factors influencing outcome in patients with bacterial brain abscess

We clearly determined the key to managing patients with brain abscess by retrospectively evaluating the factors affecting poor outcome in these patients. This study included 113 patients with brain abscess diagnosed in the CT era. Basic characteristics and therapeutic parameters were estimated as independent predictors of poor outcome by using univariate and multivariate logistic regression analysis. Patients with poor outcomes more frequently had deeply-located abscesses (p < 0.02), IVROBA (intraventricular rupture of brain abscess (p < 0.001) and were in a severely deteriorated neurological state (p < 0.001) than those with good outcomes. Multiple logistic regression analysis predicted that IVROBA (ORs, 24.5; 95% CI, 3.04 to 197.9) and severely deteriorated cases (ORs, 13.7; 95% CI, 2.34 to 80.8) resulting from IVROBA increased the relative risk of poor outcome. Patients with IVROBA more frequently had also deeply-located abscesses (p < 0.005), positively immunocompromised states (p < 0.05) and were in a severely deteriorated condition (p < 0.003) than those without IVROBA. Patients with metastatic abscess had also IVROBA (p < 0.006). Multiple logistic regression analysis anticipated that deeply-located abscess (ORs, 3.90; 95% CI, 1.38 to 11.04), and metastatic abscess (ORs, 12.26; 95% CI, 1.35 to 111.2) increased the relative risk of IVROBA. Patients in an obtunded state and with marked neurological deficit had IVROBA more often than patients in an alert state and/or mild neurological deficit (ORs, 3.23; 95% CI, 1.17 to 8.86, p < 0.03) before treatment. Our findings suggest that IVROBA strongly influences poor outcome in patients with brain abscess. The key to decreasing poor outcomes may be the prevention and management of IVROBA, by evaluating intracranial pressure pathophysiology. IVROBA should be aggressively treated by aspiration methods for the abscess coupled with appropriate intravenous and intrathecial administration of antibiotics.

A case of Graves' disease associated with autoimmune hepatitis and mixed connective tissue disease

The patient was a woman of forty-eight. Liver dysfunction was pointed out at the age of forty-five. She was admitted to hospital because of her hyperthyroidism. Her palmar skin was wet and her fingers were swollen like sausages. She had a diffuse and elastic hard goiter with a rough surface. The serum levels of free T3 (9.6 pg/mL) and free T4 (3.76 ng/dL) were high and that of TSH (0.11 microU/mL) was low. The activity of TSH-binding inhibitory immunoglobulin (TBII) was 89%. The uptake rate of 123I to the thyroid was 55.1% and the uptake pattern was nearly diffuse. The goiter was proved to contain several nodules by ultrasonography, but aspiration cytology showed no malignant cells. She was diagnosed to have Graves' disease with adenomatous goiter. She also had high ALT (34 IU/L) and gamma-globulin (1.97 g/dL). She had positive antinuclear antibody (speckled type), positive anti-ribosomal nuclear protein antibody, and positive LE cell phenomenon. The liver biopsy revealed mononuclear cell infiltration with fibrosis in the portal area. These data indicated that she also had autoimmune hepatitis (AIH) and mixed connective tissue disease (MCTD). The analysis of human leukocyte antigen (HLA) showed positive A11 which had been reported to relate to Graves' disease, and positive DR4 which had been reported to relate to AIH and MCTD. These results suggested that HLA would determine susceptibility to three distinct autoimmune diseases in this case.

Carbon beam dosimetry intercomparison at HIMAC

To verify international uniformity in carbon beam dosimetry, an intercomparison programme was carried out at the heavy ion medical accelerator (HIMAC). Dose measurements with ionization chambers were performed for both unmodulated and 6 cm modulated 290 MeV/nucleon carbon beams. Although two different dosimetry procedures were employed, the evaluated values of absorbed dose were in good agreement. This comparison established a common framework for ionization chamber dosimetry between two different carbon beam therapy facilities.

[Sedative and hypnotic properties of propofol during epidural or spinal anesthesia]

The sedative and hypnotic properties of propofol were studied in 40 patients undergoing elective gynecologic or orthopedic surgery using epidural (epidural group, n = 20) or spinal (spinal group, n = 20) anesthesia. Patients were given a bolus dose of 1 mg.kg-1 immediately followed by a continuous infusion of 4 mg.kg-1.h-1 of propofol after epidural or spinal anesthesia. The patients were asked to open their eyes on command at 10-sec interval from the end of the bolus infusion of propofol. We administered oxygen when SpO2 decreased below 93%. The mean induction time to reach the level of sedation, no response to verbal command to open their eyes, was 6 +/- 10 (SD) min in the epidural group, and 7 +/- 17 min in the spinal group. The mean recovery times, judged by ability to open eyes and recall date of birth, were 4 +/- 4 min and 7 +/- 6 min in the epidural group, and 4 +/- 4 min and 5 +/- 5 min in the spinal group, respectively. There were no differences in all recorded times between the two groups. Oxygen inhalation was needed in 75% of the patients in the epidural group and 100% in the spinal group. These results indicate that bolus and continuous infusion of propofol produces rapid induction, deep level of sedation, and rapid recovery during epidural or spinal anesthesia. However, respiratory depression appeared at this infusion rate during regional anesthesia.

Jejunal lipomatosis with diverticulosis: report of a case

We herein report the very rare case of a 68-year-old Japanese man with multiple jejunal lipomatosis and diverticulosis. He was admitted to our hospital with the chief complaint of melena and anemia. A barium study of the small bowel showed multiple lipomatosis and diverticulosis. An approximately 200-cm length of the jejunum was therefore resected. Thereafter, two diverticula and 215 lipomas were recognized in the resected specimen. A pathological examination showed mature adipose tissue with fibrous septa in the submucosal and muscularis propria. These findings were thus suggested to be due to the attenuation of the muscularis propria. The complications of lipomatosis are also discussed.

Involvement of cucumber mosaic cucumovirus RNA2 and RNA3 in viral systemic spread in radish plant

The genetics of cucumber mosaic cucumovirus (CMV) and the pathogenicity of the virus for Raphanus sativus were analyzed using pseudorecombinants constructed from the infectious transcripts of two naturally occurring strains of cucumber mosaic cucumovirus (CMV-D8 and CMV-Y). CMV-D8, but not CMV-Y, could cause systemic infection of the plant. Viral accumulation and systemic movement in the plants was examined using immuno-tissue blot analysis, dot blot and Northern blot hybridization. Virus was equally distributed and CMV RNAs accumulated to similar levels in the inoculated cotyledons of radish irrespective of the pseudorecombinant, suggesting that there are no apparent differences in the ability of infection and viral accumulation between CMV-D8 and CMV-Y. We found, however, that both RNAs 2 and 3 of CMV-D8 are involved in determining the efficiency for the systemic infection of R. sativus. Co-operated interactions between genetic information of RNAs 2 and 3 would control the efficient translocation of virus from the inoculated leaves to the uninoculated upper leaves of radish plant.

Atrial ablation with an IRK-151 infrared coagulator

BACKGROUND

The purpose of this study was to develop a method of atrial ablation. In the IRK-151 infrared coagulator, light from a tungsten-halogen lamp is focused into a quartz rod. The distal exit plane is connected to a tip made of sapphire to allow linear ablation.

METHODS

Thirty-six lesions were created in 9 mongrel dogs. The beating ventricular myocardium was ablated from the epicardium. In each dog, 4 lesions were created by using the following durations of application: 3, 9, 15, and 21 seconds. After the ablation, the myocardium was fixed and stained. A linear lesion on the beating right atrial free wall was created. Before and after the ablation, epicardial plaque-electrode mapping was performed. Three months after ablation, remapping was performed.

RESULTS

The ablated myocardium had well-demarcated necrosis without carbonization or vaporization. The maximum depth was 10.3 +/- 0.8 mm. The conducting pathway was blocked. The block, once made, continued for 3 months.

CONCLUSIONS

The IRK-151 produces well-demarcated lesions that were deep enough for atrial ablation to block the conducting pathway.

Nodal T-cell lymphoma in an HTLV-I-endemic area: proviral HTLV-I DNA, histological classification and clinical evaluation

Adult T-cell leukaemia/lymphoma (ATLL) is a human malignancy associated with human T-cell leukaemia virus type I (HTLV-I). The histology usually indicates a pleomorphic type, but is not consistent. To clarify the relationship between the histological classification and prognosis in ATLL, and to confirm the significance of clonal HTLV-I integration, we reclassified 572 cases with nodal T-cell lymphoma in which the T-cell phenotype and/or genotype was confirmed. In all cases the clonal integration of HTLV-I proviral DNA in the lymph nodes was examined by Southern blot analysis. In addition, anti-ATL antigen (ATLA) determination in the serum or PCR analysis of HTLV-I pX amplification in lymph nodes was also performed. 66/313 (21%) cases with ATLA had no evidence of clonal HTLV-I integration. 572 cases were classified into three groups: (A) cases with clonal integration (247 cases), (B) cases with ATLA without clonal integration of HTLV-I proviral DNA (66 cases), (C) cases without ATLA (259 cases). Histologically, groups B and C frequently demonstrated large cell type and angioimmunoblastic lymphadenopathy with dysproteinaemia (AILD) type; however, group A tended to show a pleomorphic type. Clinically, group A showed a poorer prognosis than groups B and C. In conclusion, group A cases were defined as ATLL (HTLV-I-associated T-cell lymphoma), whereas group B was classified as T-cell lymphoma, which had coincidently occurred in HTLV-I infected carriers. The simplified classification of REAL indicated clinical outcome: the prognosis of ATLL was poor, the unspecified type was intermediate, whereas the other types of lymphoblastic, AILD and anaplastic large cell type were all relatively favourable.

Alpha-galactosidase transgenic mouse: heterogeneous gene expression and posttranslational glycosylation in tissues

We produced six transgenic mouse lines expressing human alpha-galactosidase (alpha-Gal) in order to evaluate its posttranslational modification. Among them, serum alpha-Gal activity increased 3000-fold in two transgenic mouse lines (TgN2 and TgN51), as compared to that in non-transgenic lines. The heart and liver of the TgN2 mouse expressed a high amount of transcript as well as high alpha-Gal activity. Its gene products in the heart and kidney were sensitive to endoglycosidase H digestion, but those in the spleen and liver were largely resistant. Glycopeptidase F treatment confirmed an identical molecular mass for the peptide moiety of the enzyme. We concluded that heterogeneous molecular mass of the gene products was caused by different degrees of posttranslational glycosylation in murine tissues.

Role of carboxyl residues surrounding heme of human cytochrome b5 in the electrostatic interaction with NADH-cytochrome b5 reductase

To identify the cytochrome b5 residues responsible for the electrostatic interaction with NADH-cytochrome b5 reductase (b5R), we prepared and characterized the cytochrome b5 mutants in which Glu41, Glu42, Glu63, Asp70, and Glu73 were replaced by Ala, utilizing site-directed mutagenesis and the expression system for cytochrome b5 in Escherichia coli. Apparent Km values of the wild type b5R for Glu42Ala cytochrome b5 and Asp70Ala cytochrome b5 were approximately three-fold and six-fold higher than that for the wild type cytochrome b5, respectively, while the kcat values for those mutants were not remarkably affected. In contrast, Glu41Ala, Glu63Ala, and Glu73Ala cytochrome b5 showed almost the same kinetic properties as the wild type cytochrome b5. Furthermore, kinetic studies on combinations of the cytochrome b5 and b5R mutants suggested the interaction between Glu42 and Asp70 of cytochrome b5 and Lys125 and Lys41 of b5R, respectively, in the reaction.

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 studied by site-directed mutagenesis

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 was studied by site-directed mutagenesis. The target residues for mutagenesis were selected on the basis of the previously reported chemical cross-linking study of these two proteins, which implicated possible charge-pair interactions between Lys-41, Lys-125, Lys-162, and Lys-163 of the enzyme, and Glu-47, Glu-48, Glu-52, Glu-60, Asp-64 (group A), and heme propionate of cytochrome b5. Mutant reductases that lost one of the above-listed Lys residues showed higher K(m) values for cytochrome b5 and lower kcat values than those of the wild type, suggesting that all of the examined Lys residues participate in binding with cytochrome b5 as reported previously. In contrast, a removal of one of (or even all of) the group A residues from cytochrome b5 by mutagenesis caused no significant effect on the catalytic properties of cytochrome b5. Additional elimination of another set of negative residues (Glu-41, Glu-42, Asp-57, and Glu-63 (Group B)), which are also located close to heme, elevated the K(m) value by more than five folds. These results suggest that there should be other acidic residue(s) than group A in cytochrome b5 which participate in binding with NADH-cytochrome b5 reductase.

[Open heart surgery in patients with a tracheostoma]

From April 1992 to May 1997 six patients underwent open heart surgery, who had tracheostoma at the time of operation. The sternum was divided completely in three patients whose tracheostoma lay highly on the neck, and it was cut transversely on the manubrium at the level of the first intercostal space, below which it was split longitudinally in two patients (partial median sternotomy). In one patient right anterolateral thoracotomy was used. There were no operative death and no complication related to infection. A left internal thoracic artery (LITA) was used successfully for a bypass conduit in two patients who underwent partial median sternotomy. Dissection of the proximal portion of the LITA through the second intercostal space prior to the sternotomy made the graft procurement feasible in this particular situation. In conclusion, full-length sternotomy is performable safely when the tracheostoma lies highly on the neck, and the partial sternotomy up to the midmanubrium is applicable, including LITA harvesting, even if it is just at the sternal angle.

[The effects of flumazenil or bicuculline on the respiratory depression by morphine]

The effects of intravenous administration of flumazenil (n = 6) or bicuculline (n = 6) on the discharge of the phrenic nerve were studied following vagotomy in pentobarbital anesthetized mechanically ventilated rats. Morphine (0.4 mg.kg-1.min-1) was administrated until the respiratory rate decreased to about a half of the baseline respiratory rate. In this state, we first administered flumazenil (0.25 mg.kg-1) or bicuculline (0.4 mg.kg-1), intravenously and then administered naloxone (0.02 mg) intravenously in the two groups. The increase of inspiratory time from 0.7 +/- 0.1 to 2.0 +/- 0.5 s by morphine recovered to 0.8 +/- 0.2 s by bicuculline and to 0.6 +/- 0.1 s by naloxone. The increase of inspiratory time from 0.7 +/- 0.1 to 1.7 +/- 0.3 s by morphine, and to 2.1 +/- 0.5 s by flumazenil recovered to 0.6 +/- 0.1 s by naloxone. Expiratory time did not change during each drug administration in the two groups. The decrease of respiratory rate from 44 to 23 +/- 4 breaths.min-1 by morphine recovered to 37 +/- 5 breaths.min-1 by bicuculline and to 42 +/- 2 breaths.min-1 by naloxone. The decrease of respiratory rate from 45 +/- 3 to 22 +/- 6 breaths.min-1 by morphine, and to 18 +/- 4 breaths.min-1 by flumazenil recovered to 46 +/- 3 breaths.min-1 by naloxone. Amplitude of integrated phrenic nerve discharge increased to 125 +/- 42% by bicuculline and to 175 +/- 93% by naloxone compared to the baseline values. The decrease of amplitude to 54 +/- 18% by flumazenil recovered to 125 +/- 42% by naloxone. These results suggest that bicuculline not flumazenil antagonizes the respiratory depression of morphine by increasing the respiratory rate and respiratory movement.

Quantification of concanavalin A binding to rat brain microsomal membranes detected by fluorescence polarization technique

A high sensitive method for detecting the change of microsomal membrane surface oligosaccharides was developed to study the regulatory role of lipid- or peptide-linked mannoside of endoplasmic reticulum in synaptic functions. The binding of concanavalin A to the microsomal membrane surface was measured quantitatively using a microgram-order of rat brain microsomal proteins. The fluorescence polarization of concanavalin A (Con A)-fluorescein isothiocyanate (FITC) conjugate bound to the membrane was analyzed to quantitate the change of binding constant and the number of binding sites. As a control, the non-specific binding of bovine serum albumin-FITC conjugate was measured by the same technique. We measured the change of fluorescence intensity of membrane-bound FITC conjugates by the flow cytometry and found that the intensity of FITC conjugate bound to the membrane increased more than that of free form of the probe. We observed that the alpha-mannosidase-treatment of rat brain microsomes resulted in the increase of binding constant of Con A to the microsomal surface without significant loss of binding sites.

Coronary artery bypass grafting by median sternotomy in patients with a tracheostoma

The approach to the heart for open heart surgery in patients with a tracheostoma requires careful consideration. The presence of a tracheostoma interferes with the standard sternotomy and increases the risk of mediastinitis. We have successfully performed coronary artery bypass grafting in two patients with a tracheostoma using the limited median sternotomy and describe the surgical procedure used in these patients.

Extrachromosomal unequal homologous recombination and gene conversion in simian kidney cells: effects of UV damage

Shuttle plasmid vectors containing the SV40 origin of replication and tandem neo genes with distally placed non-overlapping deletions were used to study the effects of DNA damage on extrachromosomal homologous recombination in simian kidney cells. DNA was introduced into COS7 cells by a lipofectin-mediated transfection procedure and recombination was assessed by analyzing the structure of plasmids. Recombinational events observed included unequal homologous recombination (triplication), gene conversion, double reciprocal recombination, deletion (pop-outs), gene amplification (4-6 copies), and multimerization. Triplication, an event that previously had not been reported in association with extrachromosomal recombination, predominated in experiments with undamaged vectors. The recombination frequency (NeoR/AmpR) of vectors randomly damaged by UV irradiation was essentially unchanged; however, the relative number of triplication events decreased significantly. Selective damage in one of the two neo genes increased the relative frequency of gene conversion. The experimental system developed for use in this study detects all major homologous recombination events observed in chromosomal direct repeat sequences in mammalian cells and yeast and should prove valuable for future studies of homologous recombination in mammalian cells.