CD3D and severe combined immunodeficiency: The loss of CD3δ, -ε or -ζ results in almost absent T-cell development [3], presenting as severe combined immunodeficiency (SCID) characterized by a profound susceptibility to opportunistic infections, failure to thrive, and death within the first 2 years of life unless the patients undergo hematopoietic stem cell transplantation (HSCT) [4].