Immunological studies reveal mild or strong lympho- and leucopaenia in nearly half of the patients with an evident decrease of immunocompetent cells [75], T cell immunity in NBS patients is characterized by a reduced absolute number of CD3+ T cells in the vast majority of patients, accompanied by a reduced number of CD4+ (helper) T cells (in about 90% of patients), with naive CD4+ T cells expressing the CD45RA isoform significantly decreased or practically absent. The gene discussed is CD4; the disease is Nijmegen breakage syndrome.