Class Ib genes constitute the majority of MHC class I loci. These mono morphic or oligomorphic molecules have been described in many organisms; they are best characterized in the mouse, which contains a substantial number of potentially intact genes. Two main characteristics differentiate class Ib from class Ia molecules: limited polymorphism and lower cell surface expression. These distinguishing features suggest possible general izations regarding the evolution and function of this class. Additionally, class Ib proteins tend to have shorter cytoplasmic domains or in some cases may be secreted or may substitute a lipid anchor for the trans membrane domain. Some are also expressed in a limited distribution of cells or tissues.

At least six mouse MHC class Ib molecules have been shown to present antigens to r: x. f3 or yfJ T cells. Recent advances have provided insight into the physiological function ofH-2M3" and have defined the natural peptide binding motif of Qa-2. In addition, significant progress has been made toward better understanding of other class Ib molecules, including Qa-l, TL, HLA-E, HLA-G, and the MHC-unlinked class I molecule CDl. We begin this review, however, by arguing that the dichotomous cate gorization of MHC genes as class Ia and Ib is conceptually misleading, despite its historical basis and practical usefulness. With these reservations