Transcriptional factor that activates insulin gene expression (PubMed:12368292, PubMed:15665000). Acts synergistically with NEUROD1/BETA2 and PDX1 (PubMed:15665000). Binds the insulin enhancer C1/RIPE3b element (PubMed:12917329, PubMed:14680841, PubMed:14973194, PubMed:15665000). Binds to consensus TRE-type MARE 5'-TGCTGACTCAGCA-3' DNA sequence (By similarity). Forms homodimers (By similarity). Interacts with NEUROD1 and PDX1 (PubMed:15665000). May interact with MAFB, FOS, JUN and PCAF (By similarity). Expressed in brain, lung, spleen, pancreas and kidney (PubMed:12368292, PubMed:14680841). In the pancreas, expressed in the insulin-producing beta-cells of the islets of Langerhans (at protein level) (PubMed:12917329, PubMed:15923615). Also expressed in the eye (PubMed:12368292, PubMed:15923615). In the developing pancreas, expressed exclusively in the insulin-positive cells from 13.5 dpc onward and never in the glucagon-expressing cells (at protein level) (PubMed:14973194). At 12.5dpc, at the mRNA level, detected in each formed somite, in myotomal cells. Also detected in the head neural tube, liver cells and, at low levels, in some mesenchyme-like cells (PubMed:14680841). Up-regulated by glucose in pancreatic beta-cell lines. Ubiquitinated, leading to its degradation by the proteasome. Phosphorylated at tyrosines. Mice are born at the expected Mendelian rate and survive until adulthood. They show a normal pancreatic morphology at birth. A 12 weeks, they exhibit a reduction in the proportion of Langerhans islet beta-cells and impaired glucose-stimulated insulin secretion and eventually they develop diabetes mellitus. Belongs to the bZIP family. Maf subfamily.