The best studied of the immediate early genes is the c-Fos proto-oncogene. Many of the signals inducing Fos expression act through a sequence located in the 5' flanking region of c-Fos, designated the serum response element (SRE). The SRE is required for response to activators of protein kinase C and Fos growth-induced signals independent of protein kinase C. Accumulating evidence argues that the SRE is a multifunctional element that may involve the action of multiple SRE-binding proteins. These include the serum response factor (SRF) and the two less well characterized proteins, TCF p62 and BBF p62. An SRE binding nuclear protein, designated SRE-ZBP, is a member of the C2H2 zinc finger family of proteins. Like c-Fos, SRE-ZBP is serum-inducible in HeLa cells, although with slower kinetics.