CD19 and acute lymphoblastic leukemia: More recently, one mechanism through which resistance to CART19 therapy is mediated in ALL has been shown to be the selection of rare CD19 splice variants which retain functional CD19 antigen but lack the epitope recognized by the CAR construct.68 These observations highlight the importance for monitoring antigen expression in target tissues during treatment using high sensitivity molecular strategies that can identify at the molecular level treatment-relevant molecular alterations.