PTPROt encodes an active tyrosine phosphatase. The enzymatic activities of the PTPROt-GST fusion protein alone (▴), PTPROt-GST in the presence of 1 mmol/L sodium orthovanadate (▪), and GST alone (•) were measured using the synthetic phosphotyrosyl substrate, END(pY)INASL. PTPROt-GST dephosphorylated the indicated substrate in a concentration-dependent manner; PTPROt phosphatase activity was also markedly reduced in the presence of the PTP inhibitor, sodium orthovanadate. The control GST protein has no detectable phosphatase activity. The data are expressed as the mean ± SD from 3 independent assays.