Epidermal growth factor (EGF) receptor over-expressing, p53-deficient A431 cells response to toxic dose of EGF by G1 arrest and apoptosis. Applying cDNA expression array technology we demonstrated that EGF increased the levels of Gadd45 mRNA. Northern blot and Western blot analyses confirmed that both Gadd45 mRNA and protein were increased. Concurrently half-lives of Gadd45 mRNA and protein also increased. Nuclear run-on experiments did not show a large increase of Gadd45 mRNA transcription rate. Gadd45 mRNA and protein started to increase after 1 h of EGF treatment and remained high for up to 10 h. We have also confirmed previous studies which showed that in EGF-stimulated A431 cells p21(Cip1/Waf1) (cyclin-dependent kinase interacting protein 1) was up-regulated within the same time frame. Thus it appears that in addition to inducing G2 arrest by directly disrupting Cdc2/Cyclin B1 complex in genotoxic-stressed cells, Gadd45 may also participate in G1 arrest in growth factor overexposed cells.