c-MAF, originally identified as a homolog of the avian viral oncogen v-maf, belongs to the family of AP-1 basic leucine zipper transcription factors. It forms homodimers or heterodimers with other family members and regulates target genes through binding to the consensus Maf recognition element (MARE) sequence. c-MAF deficient helper T cells have profound defects in producing IL-4 and in differentiating into Th2 cells. Reciprocally, overexpression of c-MAF results in production of IL-4 and attenuated Th1 differentiation. These findings indicate that c-Maf is crucial for IL-4 gene expression and regulation of Th2 differentiation. In addition, studies have also shown that c-Maf is required for the differentiation of Th17 cells as well as the production of IL-10. High expression levels of c-MAF were detected in numerous multiple myeloma cell lines, suggesting that c-MAF might function as an oncogene in multiple myeloma.