CD56, an isoform of the neural cell adhesion molecule (NCAM), is a 220kD heavily glycosylated type I transmembrane protein. On human hematopoietic cells, its expression is restricted to NK cells and a subpopulation of T lymphocytes (known as large granular lymphocytes) which demonstrate natural killer activity. While it is clear that NCAM plays a role in homotypic adhesion of neuronal cells, the significance of CD56 expression on NK cells is not clear and remains controversial.