As CD99 expression levels were determined in malignant T-ALL cell lines including Jurkat E6.1 and MOLT-4 and a T-LBL cell line SUP-T1, as well as human PBMCs using mAb MT99/3 [14], in this study, CD99 expression on these cells was confirmed by indirect immunofluorescence staining using HuMT99/3. This evidence concerns the gene CD99 and acute lymphoblastic leukemia.