The Phospho-Rel(Ser503) Colorimetric Cell-Based ELISA Kit provides a lysate-free, convenient, and high-throughput approach for sensitive detection of Phospho-Rel(Ser503) protein expression profile in cells. The kit can be used for measuring the relative amounts of Phospho-Rel(Ser503) in cultured cells as well as screening for the effects that various treatments, inhibitors (ie siRNA or chemicals), or activators have on Phospho-Rel(Ser503).