FOSL1 (FOS Like 1, AP-1 Transcription Factor Subunit) is a protein-coding gene that produces the Fos-related antigen 1 (FRA1) protein, a key component of the AP-1 transcription factor complex. It is heavily involved in regulating cell proliferation, differentiation, tumorigenesis, and metastasis, particularly in cancers like lung, pancreatic, and breast cancer. FOSL1 forms heterodimers with JUN family proteins to form the AP-1 complex, which binds to DNA to regulate gene expression. It is considered an oncologist in many solid tumors, driving metastasis, EMT, and resistance to chemotherapy and radiotherapy. High expression of FOSL1 is associated with poor prognosis and tumor aggressiveness. It is often found in the stroma of reproductive tissues, cartilage, and in tumor microenvironments.