The recombinant DNA technology is a useful tool to characterize the insulin gene and adjacent areas. A highly polymorphic region near the human insulin gene was detected and its possible relation to certain types of diabetes mellitus is discussed. In three cases the synthesis of a structurally abnormal insulin of lower biological activity leads to hyperglycemia. These mutant insulin gene sequences can be identified with the help of restriction endonuclease cleavage analysis. Insulin gene expression is regulated mainly at translational level. Elements near or within the insulin gene seem to be required for a sufficient and cell-specific expression. But the role of the polymorphic locus is not yet clear. First results on gene transplantation by administration of liposome entrapped insulin gene sequences are surprising, but at this time only of speculative value for medical research.