Even though blood CD4+ T-cell levels seem to less accurately reflect the risk of PCP in HIV-negative patients compared to alveolar CD4+ T lymphocyte counts [20], CD4+ T-cell lymphopenia has been also associated with PCP in HIV-negative patients [102], particularly in hematopoietic stem cell transplantation recipients [103], in solid-tumor patients receiving chemotherapy [84], in those with an autoimmune disease, and those with a hematological malignancy [104]. This evidence concerns the gene CD4 and hematologic disorder.