Cox et al. utilized pediatric leukemia patient samples in engraftment studies with NOD/SCID mice and demonstrated the ability of CD34+/CD4− and CD34+/CD7− subfractions to populate leukemia after serial passaging.61 In contrast, Gerby et al. reported that the CD34+/CD7- subpopulation isolated from the peripheral blood of T-ALL patients contained normal human progenitors that underwent normal hematopoietic differentiation in vitro and in vivo. Here, CD34 is linked to leukemia.