Both alleles of the insulin gene of a patient with mild diabetes [maturity-onset-diabetes-of-the-young (MODY)-type syndrome] associated with hyperinsulinemia have been cloned, and the sequences have been determined. One allele contained a mutation (single nucleotide transition) in the coding sequence for the B chain at position 24 (TTC leads to TCC), resulting in the loss of a restriction enzyme (Mbo II) cleavage site in the gene. This mutation results in the substitution of serine for phenylalanine in a critically important region of the insulin molecule that is intimately involved in receptor binding. Both insulin alleles were of the alpha type and, aside from a single nucleotide deletion in the 5' region of the normal allele, their sequences were identical to those previously determined.