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Individuals in some species prefer mates carrying dissimilar genes at the major histocompatibility complex (MHC), which may function to increase the MHC or overall heterozygosity of progeny. Here I review the evidence for MHC-dependent mating preferences from recent studies, including studies on the underlying olfactory mechanisms and evolutionary functions. Many studies indicate that MHC genes influence odour, and some work is beginning to examine the potential role of MHC-linked olfactory receptor genes in mating preferences. MHC-dependent mating preference increases the MHC-heterozygosity of progeny, which is suspected to confer resistance to infectious diseases. In humans, heterozygosity at MHC loci is associated with increased resistance to hepatitis and HIV infections, but experimental evidence for the heterozygote advantage hypothesis has been lacking. Here I re-analyse data from previously published experimental infection studies with mice. I show that although overdominance is rare, resistance is often dominant, suggesting that heterozygotes are often protected. A second (nonmutually exclusive) possibility is that MHC-disassortative mating preferences promotes inbreeding avoidance. This hypothesis is supported by recent evidence that MHC genes play a role in kin recognition, and that mating with close kin has rather deleterious fitness consequences. In conclusion, I discuss other ways that MHC genes might influence sexual selection. The research on MHC-mediated mating preferences is integrating the study of animal behaviour with other seemingly disparate fields, including sensory biology and immunogenetics.