['To elucidate effects of histone deacetylase inhibitors on cell cycle of leukemia cell lines and investigate its molecular mechanisms.', 'Kasumi-1, U937 and NB4 cell lines were exposed to a histone deacetylase inhibitor, phenyl butyrate (PB), for 24, 48 and 72 hrs. Cells were harvested for cell cycle analysis by flow cytometry. Gene expression of p21WAF1/CIP1, a cyclin-dependent kinase inhibitor, was determined by semi-quantitative reverse transcriptase polymerase chain reaction (semi-quantitative RT-PCR). Promoter activity of p21WAF1/CIP1 was determined by luciferase-reporter assay in 293T cell line.', 'PB inhibited cell cycle of Kasumi-1, U937 and NB4 cell lines, showing G(0)/G(1) phase arrest and S-phase fraction reduction with a dose and time dependent manner. After Kasumi-1, U937 or NB4 cells exposed to 3 mmol/L PB for 72 hrs, G(0)/G(1)-phase fraction was increased by 42.03%, 44.36% and 26.82%, and S-phase fraction was decreased by 31.86%, 38.9% and 26.77%, respectively. After Kasumi-1, U937 and NB4 cell lines exposed to PB, the expression of p21WAF1/CIP1 gene was increased by (2.06 +/- 0.27), (2.78 +/- 0.40) and (1.78 +/- 0.20) times at its maximum, respectively. PB could stimulate p21WAF1/CIP1 promoter activity (by luciferase-reporter assay) and the effect was dose dependent. The promoter activity was increased by 5.74 times after the cells exposed to 3 mmol/L PB for 48 hrs. PB stimulating p21WAF1/CIP1 promoter activity was mainly mediated by a 101 base pairs fragment upstream of transcription start site.'] Does [ Phenol butyrate inhibit cell cycle of leukemia cell lines through up-regulation of p21WAF1/CIP1 gene ]?