Background：

This gene encodes a common acute lymphocytic leukemia antigen that is an important cell surface marker in the diagnosis of human acute lymphocytic leukemia (ALL). This protein is present on leukemic cells of pre-B phenotype， which represent 85% of cases of ALL. This protein is not restricted to leukemic cells， however， and is found on a variety of normal tissues. It is a glycoprotein that is particularly abundant in kidney， where it is present on the brush border of proximal tubules and on glomerular epithelium. The protein is a neutral endopeptidase that cleaves peptides at the amino side of hydrophobic residues and inactivates several peptide hormones including glucagon， enkephalins， substance P， neurotensin， oxytocin， and bradykinin. This gene， which encodes a 100-kD type II transmembrane glycoprotein， exists in a single copy of greater than 45 kb. The 5’ untranslated region of this gene is alternatively spliced， resulting in four separate mRNA transcripts. The coding region is no