CD38 and leukemia: After chemotherapy, relapse is determined by the persistence of chemo-refractory, quiescent leukemia-inducing cells (LI-C), also referred to as “leukemia stem cells” (LSC), originally identified retrospectively by means of serial transplantation in immunodeficient mice in a lineage-negative/CD34+/CD38− compartment [75,76,77,78].