CD33 is a transmembrane receptor expressed on cells of myeloid lineage. It is usually considered myeloid-specific, but it can also be found on some lymphoid cells.It binds sialic acids, therefore is a member of the SIGLEC family of lectins.

The myeloid antigens CD13 and CD33, which can be recognized by the monoclonal antibodies My7 and My9, respectively, are expressed by human monocytes, promyelocytes, myeloid blasts, some acute undifferentiated leukemias, and occasionally by acute lymphoblastic leukemias. By Southern blot analysis of DNA from human-hamster hybrid cells, Peiper et al. (1987) assigned CD33 to human chromosome 19. Monoclonal antibodies of the CD33 cluster group recognize a 67-kD protein, designated p67. Peiper et al. (1988) sublocalized the CD33 gene to 19q13.3 by in situ hybridization.