Acid phosphatase locus 1 (ACP1) is a low molecular weight protein tyrosine phosphatase that has been shown to be an important regulator of insulin receptor signaling. The activity assay of recombinant mouse ACP1 was measured by its ability to cleave a peptide substrate, 4-Nitrophenyl?phosphate disodium (PNPP). The reaction was performed in 50 mM NaOAc, pH 5.0 ( assay buffer), initiated by addition 50 μl of 3 ug/ml ACP1 (diluted by assay buffer) to 50 μL of 2 mM substrate. Incubated at room temperature for 5 minutes in the dark and add 100 μl of 0.2 M NaOH to stop the reaction. Read at a wavelength of 405 nm (top read), the specific activity of recombinant mouse ACP1 is >32000 pmol/min/μg.