T-ALLs express T-cell markers including CD3D, CD2, CD4, CD5, CD7 and CD99 on their cell surface1 and flow cytometric analysis of cell surface markers enables T-ALL classification based on the stage of differentiation of leukemic clones into pro-T, pre-T, cortical and mature T-ALL. The gene discussed is CD99; the disease is acute lymphoblastic leukemia.