The bare lymphocyte syndrome, type II (BLS II) is a rare recessive genetic condition in which a group of genes called major histocompatibility complex class II (MHC class II) are not expressed. The result is that the immune system is severely compromised and cannot effectively fight infection. Clinically, this is known as a severe combined immunodeficiency (SCID).
The genetic basis for BLSII is not due to defects in the MHC II genes themselves. The genetic basis is the result of mutations in genes that code for proteins (transcription factors) that normally regulate the expression of the MHC II genes . That is, one of the several proteins that are required to switch on MHC II genes in various cells types (primarily those in the immune system) is absent. The genes responsible were cloned by the laboratories of Bernard Mach  in Switzerland and Jeremy Boss  at Emory University in Atlanta, Georgia. Mutation in any one of four genes can lead to BLS II. The genes' names are: class II trans-activator (CIITA), regulatory factor of the X box 5 (RFX5), RFX-associated protein (RFXAP) and RFX ankyrin repeats (RFXANK; also known as RFXB).
Though BLSII is an attractive candidate for gene therapy , bone marrow transplant is currently the only treatment.