Appearance of the IDDM phenotype is thought to require a predisposing genetic background and interaction with other environmental factors. Rotter and Rimoin (1978) hypothesized that there are at least 2 forms of IDDM: a B8 (DR3)-associated form characterized by pancreatic autoimmunity, and a B15-associated form characterized by antibody response to exogenous insulin. Interestingly, the DR3 and DR4 alleles seem to have a synergistic effect on the predisposition to IDDM based on the greatly increased risk observed in persons having both the B8 and B15 antigens (Svejgaard and Ryder, 1977). Rotter and Rimoin (1979) hypothesized a combined form. Tolins and Raij (1988) cited clinical and experimental evidence to support the idea that those IDDM patients in whom diabetic nephropathy (see 603933) eventually develops may have a genetic predisposition to essential hypertension.