In ethnic Russians, MHC (HLA) was shown to be the major locus determining the predisposition to type 1 diabetes mellitus (T1DM). To map the regions linked to T1DM, families with concordant or discordant sib pairs were selected from the Russian population of Moscow. With these families, linkage to T1DM was demonstrated for CTLA4 (IDDM12, 2q32.1-q33), which codes for a T-cell surface antigen, and PDCD2 (IDDM8, 6q25-q27), which is homologous to the mouse programmed cell death activator gene. With polymorphic microsatellites, regions 3q21-q25 (IDDM9) and 10p12.2 (IDDM10) were also linked to T1DM. Case/control and family studies of the polymorphic markers from region 11p13 revealed a new T1DM-associated locus in the vicinity of the catalase gene (CAT); linkage to this locus was not reported earlier for other populations. Diabetic polyneuropathy (DPN) proved to be associated with single-nucleotide polymorphisms Ala(-9)Val (SOD2), Arg213Gly (SOD3), and T(-262)C (CAT) and with a polymorphic microsatellite of the NOS2 promoter. Hence oxidative stress, which results from hyperglycemia, increased mitochondrial production of superoxide radicals, and insufficient activities of antioxidative enzymes, was assumed to play an important part in DPN development in T1DM. Diabetic nephropathy (DN) showed no association with the antioxidative enzyme genes. However, the association was observed for the insertion/deletion (I/D) polymorphism of ACE and the ecNOS34a/4b polymorphism of NOS3, two genes involved in controlling vascular tonicity, and for the I/D polymorphism of APOB and the epsilon 2/epsilon 3/epsilon 4 polymorphism of APOE, two genes involved in lipid transport. In addition, polymorphic microsatellites of chromosome 3q21-q25 proved to be closely associated with DN. The tightest association was established for D3S1550, carriers of allele 12 or genotype 12/14 having high risk of DN (OR = 4.85 and 6.25, respectively). Region 3q21-q25 was assumed to contain a major gene determining DN development, while the other DN-associated genes mostly affect the progression of DN.