p53 is a cellular protein whose expression plays a crucial role in the regulation of cell proliferation and of neoplastic processes. p53 mRNA levels in mouse fibroblasts can be elevated in response to TPA and to serum stimulation. The promoter region of the p53 gene contains a conserved element which is highly homologous to the consensus AP1 binding site (7/8 matching bases). This AP1-like site, denoted the PF1 site, confers upon a heterologous promoter ability to respond to elevated expression of c-jun. Furthermore, the PF1 site binds protein(s) in a specific and serum-induced manner. Unexpectedly, this factor is most probably not AP1, as evident from the inability of an authentic AP1 site to compete the binding efficiently, as well as from the failure of purified AP1 to bind to the PF1 site. Hence, PF1 may be a novel AP1-related transcription factor. In addition, the 5' region of the p53 gene also contains an NF1 binding site, whose location suggests a possible regulatory role.