• However, many infants with SCID have T cells, showing slightly reduced, normal (T+ SCID) or high (T++ SCID) T cell counts: e.g., "functional" T+B+NK+ SCID in the defects of calcium channels [15], T+ (CD4+CD8-) B+NK+ SCID in the defect of ZAP70 [16], T+B-NK- SCID in the Hoyeraal-Hreidarsson syndrome [17]. Here, CD4 is linked to severe combined immunodeficiency.