CD28 and acute lymphoblastic leukemia: In the CD2-Lmo2 transgenic (Lmo2Tg) model of ETP-ALL, leukemia arises from an expanded CD4-CD8-CD44-CD25+CD28- (DN3a) thymocyte population (Figs. 1A and S1A), which contains preLSCs that display a stem-like phenotype [24–26, 29].