Idiopathic CD4 lymphocytopenia is a heterogenic and rare syndrome characterized by the detection of a persistent absolute CD4 T cells count <300 cells/mm3 (or <20% of total T cells) in more than one occasion and no evidence of HIV infection in absence of immunodeficiency or therapy associated with depressed levels of CD4 T cells. The gene discussed is CD4; the disease is immunodeficiency disease.