Process for detecting dephosphorylation of phosphorylated threonine or serine by phosphatase activity

The present invention relates to a process for detecting phosphatase activity in an immunoassay comprising the following steps: a) providing a protein or peptide comprising the sequence motif -Z-X-Y- or -Y-X-Z- wherein Z = serine or threonine X = a sequence of between 0 and 1000 amino acids which may be the same or different Y = tyrosine, serine or threonine as a substrate for a phosphatase, said protein or peptide being pre-phosphorylated at the Y and Z positions; b) incubating the protein or peptide with a phosphatase to form a protein or peptide which is dephosphorylated at the Z position; c) adding an antibody having a specificity to the peptide or protein that is phosphorylated in the Y and Z position; and d) detecting the phosphatase activity.