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Sporadic inclusion body myositis (sIBM) is generally regarded as the most frequent acquired myopathy presenting over the age of 40 years but its prevalence varies considerably in different countries and racial groups. There is strong evidence that genetic factors play a part in the pathogenesis of the disease and in Caucasians susceptibility has been linked primarily to the HLA-B8/DR3 alleles and the 8.1 MHC ancestral haplotype (AH) which is also associated with other autoimmune diseases. We have proposed that the variable prevalence of sIBM in different populations may be related to differences in the population frequency of this haplotype. Our recent observations in a large cohort of Australian sIBM cases have shown that the clinical phenotype at presentation is quite variable and have confirmed the strong link with HLA-B8/DR3 and the 8.1 AH, but have shown that the influence of the MHC is likely to be more complex than previously thought with HLA alleles also having modifying effects on the age-at-onset, disease severity and rate of progression. As a result of recent recombinant mapping studies of polymorphisms in the Class II/III regions of the MHC our group has further refined the susceptibility region in 8.1 AH associated cases and has identified a number of candidate genes that warrant further investigation. The significance of these findings for the pathogenesis of the disease will be discussed.