Dysregulated expression of Maf proteins (namely c-Maf, MafA and MafB) leads to multiple myeloma in humans and oncogenic transformation of chicken embryonic fibroblasts. Maf proteins are transcriptional activators of tissue-specific gene expression and regulators of cell differentiation. For example, MafA is a critical regulator of crystallin genes and the lens differentiation program in chickens. In mammals, MafA is essential for the development of mature insulin-producing beta-cells of pancreas. It has been shown that MafA protein stability is regulated by phosphorylations at multiple serine and threonine residues. Here, we report that Maf proteins are also post-translationally modified by small ubiquitin-like modifier (SUMO) proteins at a conserved lysine residue in the amino-terminal transactivator domain. A SUMOylation-deficient mutant of MafA (K32R) was more potent than wild-type MafA in transactivating luciferase reporter construct driven by alphaA-crystallin or insulin gene promoter. In ovo electroporation into developing chicken embryo showed that the K32R mutant induced ectopic delta-crystallin gene expression more efficiently than the wild-type MafA. We also demonstrated that the K32R mutant had enhanced ability to induce colony formation of a chicken fibroblast cell line DF-1. Therefore, SUMOylation is a functional post-translational modification of MafA that negatively regulates its transcriptional and transforming activities.