Histocompatibility antigens in diabetic identical twins.

84 pairs of diabetic identical twins were HL-A typed. 22 were maturity-onset diabetics (diagnosed after age 45); all were concordant and they showed no disturbance of HL-A frequencies. Of the remaining 62 pairs of juvenile-onset diabetics, 31 were discordant (only one twin diabetic) and 31 concordant (both twins diabetic). The frequency of the W15 antigen was equally increased in both the concordant and discordant pairs; HL-A8 was increased only in the concordant pairs. The fact that W15 is increased in the discordant pairs shows that there is genetic predisposition to diabetes even in these twins; the fact that HL-A8 is not increased suggests that different alleles may underlie susceptibility in the two groups of twins.