The mouse class I multigene family of the major histocompatibility complex (MHC) contains 25-35 homologous genes which are distinguished by organization and function (Hood et al. 1983). There are three to five H-2 genes, some of which encode the transplantation antigens, H-2K, H-2D, and H-2L, which are expressed on most somatic tissues and mediate interactions between cytotoxic T cells (CTLs) and their targets. The extraordinary polymorphism of these molecules may ensure that a proportion of the population can effectively respond to a variety of infectious agents (Klein and Figueroa 1986). The H -2 K region contains two genes arranged head to head in BALB/c (Steinmetz et al. 1982) and C57BL/10 (Weiss et al. 1984) mice. They are separated from the balance of mouse class I genes, lying centromeric to the class II and class III loci. Several nonpolymorphic class I products of more limited tissue distribution are encoded by the Qa and T/a genes of the Tla complex, located distal to the H-2D region (reviewed by Robinson 1987, Weiss 1987). The function of these molecules is unknown, although they do not appear to be involved in MHC-restricted antigen presentation (Robinson 1987).
We have undertaken a comparative sequence analysis of all of the class I genes of the C3H mouse (H-2 k haplotype) in order to elucidate the relative contributions of point mutation, recombination, and selection in the concerted evolution of the class I multigene family (reviewed in Klein and Figueroa 1986, Hughes andNei 1988). Toward this end, we screened C3H/HeN lambda (Watts et al. 1987) and C3H/HeJ cosmid (Stratagene, Ja Jolla, California) libraries with the class I cDNA pH2IIa (Steinmetz et al. 1981) and genomic pKbAi3 (Vogel et al. 1988), and identified clones corresponding to the various loci by correlating their restriction maps with those published for other inbred strains (Steinmetz et al. 1982, Weiss et al. 1984, Stephan et al. 1986). The H-2I~