Although patient history and clinical picture may strongly suggest HLA class I deficiency, diagnosis need to be confirmed by serological HLA typing (but not by molecular HLA typing because this would not identify the disease) and/or by flow cytometry. Cells from parents and siblings should be included, to identify other affected family members. The degree to wich surface expression of HLA class I molecules is reduced may already give some indications about the possible origin of the defect. HLA genotyping of the patient and his family should be performed. If the patient’s HLA genotype is homozygous, the deficiency is likely to be linked to chromosome 6 and may thus be a transporter associated with antigen processing or a tapacin deficiency whose transmission is recessive and autosomal. If not, a transcriptional and conditional defect could be envisaged and confirmed by activation of Tc ells (phytohemagglutinin, IL-2, allogenic feeder cells) and further stimulation with inflammatory cytokines, which should then strongly upregulate HLA class I molecules.