Two insulin genes of the NON mouse, an animal model of human non-obese, non-insulin-dependent diabetes mellitus, were isolated and characterized to examine the hypothesis that these genes are structurally different from those of normal mice. The NON mouse was found to have two non-allelic insulin genes, as does the normal mouse, and no structural differences were found between the normal and NON mouse in the nucleotide sequence of the insulin gene, including that of the 5'-transcriptional regulatory region, and in the deduced amino acid sequence. There was, however, an additional 113 bp sequence and seven point mutations in a further 5'-flanking region, and three point mutations in the 3'-flanking region of the insulin II gene. We conclude that reduced expression of insulin genes in the NON mouse is not due to the structural change in the known transcriptional regulatory region, although the effect on insulin II gene expression of an additional sequence upstream of the 5'-flanking region, as the negative regulatory factor, remains to be elucidated.