CR2 and acute lymphoblastic leukemia: Additionally, FACS analysis of T-ALL cell lines showed that CD21 existed on 70% of human T-ALL cell lines, and CD21 positivity varied by maturational stage, with cortical T-ALL exhibiting the highest expression (80% of cases), followed by pre-T (72%), mature (67%), ETP (25%), and pro-T (17%), suggesting that CD21 is a potential target of T-ALL [73].