CD33 and acute myeloid leukemia: Whereas tumor-specific antigen has not been identified for many tumors like some types of AML [117], and in order to prevent previously described on-target, off-tumor occurrence, two studies have demonstrated that baring the hematopoietic stem and progenitor cells from CD33 using CRISPR-Cas9 and successful transplantation of these CD33-knockout cells resulted in the safe and fully on-target, on-tumor activity of CD33-targeted CAR T-cells against AML [118, 119].