 The initial discovery of mad in Drosophila paved the way for later experiments that identified the responder to TGF-β signaling in vertebrates, called SMADs.Activated Mad is able to bind to DNA and act as a transcription factor to affect the expression of different genes in response to Dpp signaling.Genes activated by Dpp signaling include optomotor blind (omb) and spalt, and activity of these genes are often used as indicators of Dpp signaling in experiments.