In human fibroblasts, N-phosphoacetyl-L-aspartate (PALA) and γ-radiation induce reversible and irreversible p53-mediated G₁ cell cycle arrest, respectively. By coupling the premature chromosome condensation technique to fluorescence in situ hybridization, we found no evidence of DNA damage after PALA treatment. We used representational difference analysis (cDNA-RDA) to study changes in gene expression after PALA treatment and γ-radiation in normal human fibroblasts. The mammary-derived growth inhibitor (MDGI) gene was expressed in PALA-treated cells. Ectopic MDGI expression arrested PALA-treated but not irradiated RKO cells. Expression of an antisense RNA against MDGI resulted in partial G₁ escape of PALA-treated human fibroblasts. The tumor necrosis factor stimulated gene 6, TSG-6, seems to be under the control of p53 and is only and specifically induced upon PALA treatment. In irradiated cells we have identified `novel' genes that are differentially expressed, along with known genes not previously linked to cell cycle control. Some of these `novel' genes correspond to clones in the expressed sequence tag (EST) database; one of them shows identity with ESTs mapping to a region on chromosome 7, where gene(s) involved in replicative senescence and frequently deleted in tumors are located. Thus, PALA treatment and γ-irradiation elicit a pattern of differential gene expression that could contribute to a quiescence or senescence-like phenotype.