KLRK1 and juvenile myelomonocytic leukemia: Many CARs based on ligand or receptor are being tested in preclinical and clinical studies in a variety of malignancies, including FLT3 ligand to target FLT3+ AML [21], a proliferation-inducing ligand (APRIL) to target multiple myeloma expressing B cell maturation antigen [37], granulocyte macrophage colony-stimulating factor (GM-CSF) to target the GM-CSF receptor (CD116) of juvenile myelomonocytic leukemia [38] or natural killer cell receptor D (NKG2D) to target NKG2D ligands on the surface of malignant hematologic cells [39].