Unlike most cell-based thymidine analogassay kits that can only measure qualitative differences in incorporation, the CytoSelect™ IdU Competitive ELISA Kit quantifies IdU incorporated into cellular DNA. The unknown DNA samples or standards are first added to a microplate preadsorbed with a IdU conjugate. After a brief incubation an anti-IdU monoclonal antibody is added, followed by an HRP-conjugated secondary antibody. The IdU in solution and the IdU coated on the plate compete for binding of the antibody. If no IdU is in solution, all of the antibody will bind to the plate resulting in a high signal. High levels of IdU in solution will bind the antibody and be washed away, resulting in a low signal.