Human CD13 is expressed on the majority of peripheral blood monocytes and granulocytes. It is also expressed by the majority of acute myeloid leukemias, chronic myeloid leukemias in myeloid blast crisis, a smaller percentage of lymphoid leukemias and myeloid cell lines. CD13 is also found in several types of non hematopoietic cells such as fibroblasts and endothelial cells and in a soluble form in blood plasma. CD13 is not expressed on B cells, T cells, platelets or erythrocytes. In the mouse, CD13 is a non-covalently linked homodimer of approximately 150kD subunits expressed by a variety of cells including monocytes, macrophages, dendritic cell and veiled cells.