Four distinct genes encode alkaline phosphatases (APs) in humans. The ALPL gene encodes the liver/bone/kidney isozyme, also known as the tissuenonspecific AP (TNAP). In comparison, ALPI, ALPP and ALPPL2 encode intestinal, placental and placental-like or germ cell APs, respectively. The serum levels of human APs are useful tumor markers. There are many mutations in the ALPL gene, leading to different forms of hypophosphatasia, characterized by poorly mineralized cartilage and bones. The native ALPL is a glycosylated homodimer attached to the membrane through a GPI-anchor. The activity assay of recombinant mouse ALPL was measured by its ability to cleave a peptide substrate, 4-Methumbelliferly phosphate. The reaction was performed in 50 mM Tris, 1 mM MgCl2, pH 9.0 ( assay buffer), ainitiated by addition 50 μl of 0.3 ug/ml ALPL (diluted by assay buffer) to 50 μL of 50 uM substrate. Read at excitation and emission wavelengths of 365 nm and 445 nm (top read), respectively, in kinetic mode for 5 minutes. The specific activity of recombinant mouse ALPL is >11000 pmol/min/μg.