RFX5 and MHC class II deficiency: Lymphocyte major histocompatibility complex (MHC) class II deficiency also referred to as the bare lymphocyte syndrome, is a rare autosomal recessive primary combined immunodeficiency syndrome that accounts for 5% of all cases of severe combined immunodeficiency and characterized by a defective expression of human leucocyte antigen (HLA) class II molecules due to mutations in four different MHC II regulatory genes (CIITA, RFXANK, RFX5, RFXAP) [1].