The cluster of differentiation (CD) system is commonly used as cell markers in Immunophenotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules associating with the immune function of the cell. There are more than 320 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 24, also known as signal transducer CD24 or heat stable antigen CD24 (HSA), is a mucin-type glycosylphosphatidylinositol-linked glycoprotein expressed on the surface of B-cells, differentiating neuroblasts and many tumors. It is involved in molecular adhesion and metastatic tumor spread and serve as a normal receptor for P-selectin. The CD24 / P-selectin pathway could be important in disseminating tumor cells by facilitating the interaction with platelet and endothelial cells. It has also been considered as a tumor marker. High rate of CD24 expressions have been found in epithelial ovarian cancer, breast cancer, non-small cell lung cancer, prostate cancer and pancreatic cancer.