The major histocompatibility complex (MHC) is a much studied region of the mammalian genome because of its uniquely high level of polymorphism. Both natural and sexual selection have been implicated in the maintenance of MHC diversity. The elevated heterozygosity observed at the MHC could reflect overdominant heterozygote advantage against pathogenic infection. Equally, MHC disassortative mating would lead to an excess of heterozygotes.