ZAP70 is an established autosomal recessive cause of severe combined immunodeficiency (Arpaia et al. 1994; Elder et al. 1994; A. C. Chan et al. 1994) marked by an absence of CD8 + T-cells and CD4 + T-cells which do not respond to TCR-mediated activation. Here, ZAP70 is linked to severe combined immunodeficiency.