Compositions, methods, and kits for detecting and monitoring kinase or
phosphatase activity are described. The compositions typically include a
peptide, a detectable moiety, and a protease cleavage site. Modification
of a peptide by a kinase or phosphatase alters the proteolytic
sensitivity of the peptide, resulting in a change of a detectable
property of the composition. Panel assays for determining substrates or
modulators of kinase or phosphatase activity are also described.