The cluster of differentiation (CD) system is commonly used as cell markers in immunophynotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules which associating with the immune function of the cell. There are more than 32 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. Cluster of differentiation 1 (CD1), also known as Neprilysin and neutral endopeptidase, is a member of the CD system. CD1 is a zinc-dependent metalloprotease enzyme that had function to degrade a number of small secreted peptides such as the amyloid beta peptide. It exist as a membrane-bound protein and have high concentration in kidney and lung tissues. Mutations in the CD1 gene can induce the familial forms of Alzheimer's disease, providing strong evidence for the protein's association with the Alzheimer's disease process. CD1 is also associated with other biochemical processes.