Major histocompatibility complex (MHC) class I human genes have been divided into two different subgroups. The classical class I loci, human leukocyte antigen (HLA)-A, -B and -C, encode highly polymorphic membrane glycoproteins. These proteins present intracellular-derived peptides to cytotoxic T cells (1 ) and are expressed in all nucleated cells. In contrast, the nonclassical class I genes encode molecules (class Ib molecules) that have only recently been defined and their functions are not fully understood (HLA-E, -F, and -G) (2 –10 ).