FCGR2A and monocytic leukemia: K562 is a human erythroleukemia cell line, whereas U937 (human pro-monocytic leukemia cell line) and THP-1 (human monocytic leukemia cell line) are undifferentiated cells that do not have the same biological properties of primary human monocytes which express high levels of both FcγRII (CD32) and FcγRI (CD64), as well as low levels of FcγRIII (CD16).