CD4 and severe combined immunodeficiency: Near complete loss of the enzymatic activity leads to SCID but, where enzymatic activity remains, phenotypes are less severe.33 These phenotypes include not only Omenn syndrome but also leaky SCID, SCID with expansion of γδ T cells (often associated with cytomegalovirus infections), combined immune deficiency with granuloma and/or autoimmunity and idiopathic CD4+ T‐cell lymphopenia.