BP-1, also known as Ly-51 and 6C3 antigen, is a type II homodimeric transmembrane glycoprotein formed by two disulfide-linked chains of 140 kDa. Originally identified on the surface of pre-B cell lymphomas the cDNA encoding BP-1 has been cloned and found to represent aminopeptidase A. It is expressed at high levels on bone marrow stromal cell lines which support in vitro B lymphopoiesis and on a wide variety of mouse tissues known to possess aminopeptidase activity. Subsets of normal bone marrow pre-B and B cells express low levels of BP-1 which is rapidly upregulated on pre-B cells in the presence of IL-7. The BP-1 marker is commonly used to identify B cells in different stages of development.