The common acute lymphoblastic leukemia antigen (CALLA/CD10) is a single-pass type II transmembrane metallo-endopeptidase that cleaves and inactivates a variety of peptide growth factors important for signal transduction including the enkephalins, bombesin and substance P. CD10 is expressed by a number of hematopoietic cells such as immature T and B cells, B cells of the germinal centers of lymphoid follicles and granulocytes. It also reacts with a variety of non-hematopoietic cells, including epithelial cells in GI tract and kidney tubules. In liver, the bile canaliculi show a moderate to strong staining. CD10 has been used for the identification and classification of certain types of malignant lymphoma and leukemia. CD10 is expressed in a high percentage of cases of acute lymphoblastic leukemia, follicular lymphoma, Burkitt's lymphoma, some hematopoietic tumors, and chronic myelogenous leukemias in lymphoid blast crisis. It is also known to be a marker of endometrial stromal cells. It is helpful in differentiating endometrial stromal sarcoma (ESS) from uterine cellular leiomyoma (UCL) and uterine leiomyosarcoma (ULS).