The rise and fall of great class I genes.

Class I major histocompatibility complex molecules are components of the vertebrate immune system. Polymorphic classical class I molecules determine the specificity of cytolytic T cell and natural killer cell responses and are found in all species. During the timeframe of mammalian evolution, the lifetimes of a functional class I locus are short and those of individual alleles even shorter. In this seminar the role of heterozygote advantage frequency-dependent selection, disease-specific selection and drift in driving this rapid evolution is discussed. The other, non-classical genes, perform other functions within the immune system. They are of more recent invention than the classical class I genes and appear to have evolved from classical class I alleles.