The class II major histocompatibility complex antigens are cell-surface heterodimers consisting of an alpha and a beta chain. Cosmid cloning has shown that the three families of class II antigens, DR, DQ, and DP, are encoded within the HLA-D region of chromosome 6 as a series of discrete gene clusters. The DP cluster contains two pairs of alpha and beta genes, one of which encodes the biochemically-defined DP antigen. In order to assess whether the other two genes, SX alpha and SX beta, are also expressed, potential coding regions have been subcloned and sequenced. The SX3 beta gene is shown to contain regions closely homologous to all six exons of DP beta. A 1 bp deletion in the beta 2 exon, also observed for the SX4 beta allele, causes a translational frameshift, suggesting that SX beta is a pseudogene. However, all the other exons, as well as their splice sites and the putative promoter region, appear to be intact.