The ERF transcriptional repressor is a downstream effector of the RAS/ERK pathway that interacts with and is directly phosphorylated by ERKs in vivo and in vitro. This phosphorylation results in its cytoplasmic export and inactivation, although lack of ERK activity allows its immediate nuclear accumulation and repressor function. Nuclear ERFs arrest cell cycle progression in G(1) and can suppress ras-dependent tumorigenicity. Here we provide evidence that ERF function is mediated by its ability to repress transcription of c-Myc. Promoter reporter assays indicate a DNA binding-dependent and repressor domain-dependent Myc transcriptional repression. Chromatin immunoprecipitations in primary cells suggest that ERF specifically binds on the c-Myc promoter in an E2F4/5-dependent manner and only under conditions that the physiological c-Myc transcription is stopped. Cellular systems overexpressing nuclear ERF exhibit reduced c-Myc mRNA and tumorigenic potential. Elimination of Erf in animal models results in increased c-Myc expression, whereas Erf(-)(/)(-) primary fibroblasts fail to down-regulate Myc in response to growth factor withdrawal. Finally, elimination of c-Myc in primary mouse embryo fibroblasts negates the ability of nuclear ERF to suppress proliferation. Thus Erf provides a direct link between the RAS/ERK signaling and the transcriptional regulation of c-Myc and suggests that RAS/ERK attenuation actively regulates cell fate.