Clone MY31, a mouse antibody, binds to the human 140-kDa glycoprotein, an isoform of neural cell adhesion molecule (NCAM) known as CD56. CD56 and CD16 expressing lymphocytes are primarily considered as human NK cells and NK-T cells. A subset of CD56+ NK cells play a unique functional role in the innate immune response as the primary source of NK cellderived immunoregulatory cytokines, regulated in part by differential monokine production. CD56 is also expressed in normal and neoplastic human neuroendocrine tissues, certain large granular lymphocyte (LGL) leukemias, small-cell lung carcinomas, neuronal derived tumors, myelomas, and myeloid leukemias. Increased CD56 + lymphocyte subsets in peripheral blood has a significant predictive or prognostic factor in metastatic breast cancer.