Fos-related antigen 1 (FRA1) is a protein that in humans is encoded by the FOSL1 gene. The Fos gene family consists of 4 members: c-Fos, FOSB, FOSL1, and FOSL2. These genes encode leucine zipper proteins that can dimerize with proteins of the JUN family, thereby forming the transcription factor complex AP-1. As such, the FOS proteins have been implicated as regulators of cell proliferation, differentiation, and transformation. FOSL1 has been shown to interact with USF1 (human gene) and C-jun.