The CD99R (HIT4) antibody recognizes a 32-kDa type-I single chain highly glycosylated transmembrane protein. CD99R is an isoform of CD99 with restricted cell distribution. CD99R antigen is expressed on hematopoietic progenitors, thymocytes, a subset of T cells (30-42% of CD4+ cells and 15-23% of CD8+ cells), NK cells and some leukemic cells, but is not on B cells, erythrocytes and platelets. In leukemia, the co-expression of CD99R antigen and CD38 antigen predicts that the leukemic cells are in early or immature stage.