RFX-B is the gene responsible for the most common cause of the bare lymphocyte syndrome, an MHC class II immunodeficiency

The bare lymphocyte syndrome (BLS) is characterized by the absence of MHC c lass II transcription and humoral- and cellular-mediated immune responses t o foreign antigens. Three of the four BLS genetic complementation groups ha ve defects in the activity of the MHC class II transcription factor RFX. We have purified the RFX complex and sequenced its three subunits. The sequen ce of the smallest subunit describes a novel gene, termed RFX-B. RFX-B comp lements the predominant BLS complementation group (group B) and was found t o be mutant in cell lines from this BLS group. The protein has no known DNA -binding domain but does contain three ankyrin repeats that are likely to b e important in protein-protein interactions.