CD34 and neoplasm: The features for an ideal pair are: no overlapping expression in normal tissues to minimize systemic off-tumor toxicity, very low level expression in CD34+CD38- hematopoietic stem cells (HSCs) to minimize cytotoxicity, very low expression in normal resting and activated T cells to minimize T cell reactivity, expression (for the combination) in all tumor cells to overcome clonal heterogeneity, expression in LSCs, and co-expression in tumor-cells to prevent antigen escape [27].