Expression of neural cell adhesion molecules (N-CAM) provides neurons with a means of attaching to and interacting with other cells and the extracellular matrix. Alternative splicing of N-CAM mRNA results in several N-CAM isoforms. The three major isoforms are referred to as 180 kD, 140 kD, and 120 kD. Individual isoforms undergo various post-translational modifications, including the addition of polysialic acid residues to embryonic forms. The highly polysialylated forms of N-CAM present in embryos and neonates are lost during maturation and replaced by less-polysialic acid-laden mature forms of N-CAM. The CD56 antigen is a 175-185kDa isoform of the neural cell adhesion molecule (NCAM). It is expressed by lymphocytes mediating non-MHC restricted cytotoxicity and is present on some neural tissues and tumors. It is expressed on natural killer (NK) Cells and a subpopulation of T lymphocytes. NK cells make up approximately 10-25% of peripheral blood lymphocytes.