SMADs are members of the MAD-related family of molecules. MAD-related proteins 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 beta superfamily. SMADs can be divided into receptor-regulated SMADs, common-mediator SMAD and inhibitory SMADs. SMAD1, SMAD5 and SMAD9 have high degrees of homology. SMAD8 and SMAD9 are used as alternate names for the same protein. Human SMAD1 is a 465 amino acid protein, human SMAD5 is a 465 amino acid and human SMAD9 is a 430 amino acid protein.