The antigen is expressed during the early stages of T- and B-lymphocyte differentiation, is lost during the intermediate stages of maturation, and then reappears during the final stages of maturation. The antigen is also expressed in some cases of T- and B-acute lymphoblastic leukemia (ALL), Burkitt's lymphoma, multiple myeloma, and acute myeloid leukemia (AML). CD8 + CD38 + T lymphocytes are involved in the immune response to viral infection. Elevated levels of CD8 + CD38 + lymphocytes are observed in human immunodeficiency virus (HIV) infection, and during the acute stages of Epstein-Barr virus (EBV) and cytomegalovirus (CMV) infection. Increased numbers of CD8 + CD38 + T lymphocytes are also present in the period immediately following bone marrow transplantation. In HIV infection, the coexpression of the CD8 and CD38 antigens, with a concomitant low percentage of CD4 + lymphocytes, is closely associated with disease progression. Beginning at seroconversion, and during the first stages of HIV infection, the number of CD8 + T lymphocytes increases while the number of CD4 + T lymphocytes decreases. When the onset of acquired immune deficiency syndrome (AIDS) occurs, there is a decline in the absolute CD3 +, CD4 + , and CD8 + lymphocyte levels along with a further decrease in the CD4 + T lymphocyte percentage, but the relative percentage of CD8 + T lymphocytes continues to rise until, in the late stages of AIDS, the majority of the remaining T lymphocytes are CD8 +.