The proportion of CD99+ cells was significantly higher in T-ALL (median 90.5%, range 27.4–97%) than BM MNC (22%, 6.8–51.9%), HSC (22%, 10–62.7%) and CD3+ T cells (0.55%, 0.14–15.9%, P≤0.004). This evidence concerns the gene CD99 and acute lymphoblastic leukemia.