We previously described a case of familial hyperproinsulinemia, the fifth to be reported. In the present study we characterized the genetic defect carried by this family and demonstrated that it could be detected by polymerase chain reaction-single stranded conformational polymorphism. Since the serum proinsulin molecule from the propositus, a 63-yr-old Japanese man, was eluted on the same fraction of human proinsulin intermediate cleaved only at the B-C junction, we sequenced exon 3 of his insulin gene, including the C-A junction. A point mutation was discovered that changed codon 65 from arginine (CGT) to histidine (CAT) in one allele. This was the same point mutation as that described previously in three unrelated kindreds representing two races, consistent with the hypothesis that the dinucleotide sequence CpG may be a "hot spot" for mutations. Recently, developed polymerase chain reaction-single stranded conformational polymorphism proved useful in detecting this mutation in the family members. The daughter of the propositus and one of his two grandsons were also demonstrated to be heterozygous for this point mutation by this method.