The clone 5.1H11, a mouse monoclonal antibody, selectively binds to the human 140- kDa glycoprotein, an isoform of neural cell adhesion molecule (NCAM) known as CD56. It is a member of Immunoglobulin (Ig) superfamily. CD56 and CD16 expressing lymphocytes are primarily considered as human NK cells and NK-T cells. A subset of CD56+ NK cells plays 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 expressed in normal and as well as neoplastic human neuroendocrine tissues, certain large granular lymphocyte (LGL) leukemias, small-cell lung carcinomas, neuronal derived tumors, myelomas, and myeloid leukemias. Additionally, increased CD56+ lymphocyte subsets in peripheral blood are a significant predictive or prognostic factor in metastatic breast cancer.