SMADs are a family of intracellular proteins that are essential components in the signaling pathways of the serine/threonine kinase receptors of the transforming growth factor β superfamily. SMADs can be divided into receptor-regulated SMADs (R-SMADs: SMAD1, SMAD2, SMAD3, SMAD5, SMAD8, and SMAD9), common-mediator SMAD (co-SMAD: SMAD4), and inhibitory SMADs (I-SMADs: SMAD6 and SMAD7). SMAD1, SMAD5, SMAD8, and SMAD9 have high degrees of homology and antibodies are available that recognize sequences common to all of them. SMAD8 and SMAD9 are typically used as alternate names for one another in the literature.
