Combined HLA class 1 and II deficiency where a significant reduction in the surface expression of HLA class I molecules is associated with a lack of expression of HLA class II molecules. As in type II bare less lymphocyte, the resulting absence of humoral and cellular immune responses produces extreme susceptibility to viral, bacterial and parasitic infections early in life. The reduced expression of HLA class I molecules seems to be a direct consequence of the mutations of the regulatory factor X complex, which regulates the transcription of HLA class II genes. Indeed, the promoters of the genes of the heavy chains of HLA class I molecules and β2 contain binding motifs for several subunits of the regulatory factor X complex. These transcription factors thus may also play a role in the transactivation of the genes of HLA class I molecules.