The class II α and β chains were identified during the late 1970s by many groups, for both H-2 (Cullen et al. Transplantation 30:236, 1976; Freed et al. J Immunol 121:91, 1978; Cook et al. J Immunol 123:2799, 1979) and HLA-DR systems (Humpreys et al. J Exp Med 144:98, 1976; Kaufman and Strominger, Proc Natl Acad Sci USA 576:6304, 1979).