SMAD (protein)

SMAD's (Small Mothers Against Decapentaplegic) are a class of proteins that modulate the activity of transforming growth factor beta ligands.

The SMAD's form complexes, often with other SMAD's, enter the nucleus and serve as transcription factors.

Classes
There are three classes of SMAD:
 * 1)The receptor-regulated Smads(R-SMAD) which include SMAD1, SMAD2, SMAD3, SMAD5 and SMAD9
 * 2) The common-mediator Smad(co-SMAD) which include only SMAD4,
 * 3) The antagonistic or inhibitory Smads(I-SMAD) which include SMAD6 and SMAD7.

Nomenclature
The SMAD proteins are homologs of both the drosophila protein, mothers against decapentaplegic (MAD) and the C. elegans protein SMA. The name is a combination of the two.

During Drosophila research, it was found that a mutation in the gene, MAD, in the mother, repressed the gene, decapentaplegic, in the embryo. The phrase "Mothers against" was added since mothers often form organizations opposing various issues eg. Mothers Against Drunk Driving or (MADD).