We have previously shown that blasts from both BCP- and T-ALL cases can be maintained in culture for extended periods of time with no loss of karyotype or TCR rearrangements.[2, 5] Data from the long-term suspension cultures and serial NSG passage indicate that expression of CD99 is not a prerequisite for proliferation in vitro or for leukemia propagating capacity in vivo. This evidence concerns the gene CD99 and acute lymphoblastic leukemia.