Class I genes/molecules are operationally divided into two types, IA and IB. The former are represented by 1-3 genes/species which encode polymorphic molecules that present pathogenic peptides to CD8 T cells. Class IB molecules are encoded by a large family of class I related genes. In general these genes are much less polymorphic than 1A and their cell membrane expression is reduced relative to IA molecules. Since the structure of these molecules differs from class IA antigens to varying extents, the function that each class IB molecule plays in the immune system, if any. is largely unknown. Many different laboratories are studying various IB molecules including M3, Qa-2. CD-I, and TL (see articles in this issue) while our laboratories have focused on the structure and function of Qa-1. Why Qa-1? There are several features about this antigen that make it attractive to analyze. This includes the fact that it is ubiquitously expressed (Transy et al. 1987, Aldrich et al. 1988, Cochet et al. 1989), appears to have no null allele (Nakayama et al, 1990), binds ligands recognized by both a/?(Lowen et al. 1993) as well as yd T cells (Vidovic et al. 1989, Imani & Soloski 1991) and serves as a target antigen for both peptide-specific alloreactive (Aldrich et al. 1994) as well as self restricted CD8* T cells (Jiang et al. 1995). Thus, examining the structure and function of this antigen in these responses may provide insight into understanding T-ceU recognition in general and perhaps shed light on whether this molecule plays a unique role in the immune system.