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 cell-derived 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.