This setting includes HIV-infected patients who have not recovered adequate CD4 T cell counts after 1 or 2 years of HAART (the so-called “immune non-responders”), post-HSCT patients who have received a cord blood transplant or grafted with CD34+ cells (both very poor in lymphocytes progenitors), patients with rare cases of idiopathic lymphopenia, and approximately 25% of post-chemotherapy cancer patients. This evidence concerns the gene CD4 and lymphopenia.