The human major histocompatibility complex (MHC) includes within a region of 2 megabases six class I genes, of which HLA-A,-B, and-C encode the membrane-anchored heavy chains of class I molecules, which present antigenderived peptides to or [3 T cells (Bjorkman and Parham 1990). These" classical" or class Ia MHC genes are polymorphic and ubiquitously expressed, reflecting their principal role in the immune defense against intracellular pathogens (Klein and Figueroa 1986). They are distinguished from the" nonclassical" or class Ib genes, represented by HLA-E,-F, and-G in humans, which lack significant polymorphism and are nonubiquitously transcribed (Hedrick 1992; Geraghty 1993). Only HLA-G is expressed at the cell surface and may have a function in antigen presentation (Kovats et al. 1990). Among different species, class lb genes vary extensively in number and chromosomal organization, and their functional significance has been disputed (Klein and O'hUigin 1994). Evidence from the mouse indicates that some class Ib genes have been adapted to modified immune defense functions (Hedrick 1992; Teitell et al. 1994; Stroynowski and Forman 1995). Among the multiple genes in the H-2T,-Q, and-M clusters, the M3 gene encodes a class Ib molecule that selectively presents bacterial peptides to c~ T cells (Wang et al. 1991; Kurlander et al. 1992; Pamer et al. 1992), and TlOb and T22 b encode molecules recognized by"[5 T cells, apparently in a peptide-independent manner (Ito et al. 1990; Schild et al. 1994; Kaliyaperumal et al. 1995). However, these genes exist only in mice and perhaps