MyoD

MyoD is a protein with a key role in regulating muscle differentiation. MyoD belongs to a family of proteins known as myogenic regulatory factors (MRFs). These bHLH (basic helix loop helix) transcription factors act sequentially in myogenic differentiation. MRF family members include MyoD, Myf5, myogenin, and MRF4 (Myf6).

MyoD is one of the earliest markers of myogenic commitment. MyoD is expressed in activated satellite cells, but not in quiescent satellite cells. Although MyoD marks myoblast commitment, muscle development is not dramatically ablated in mouse mutants lacking the MyoD gene. This is likely to be due to functional redunancy from Myf5.

Functions of MyoD
The function of MyoD in development is to commit mesoderm cells to a skeletal lineage, and then to regulate that process. MyoD may also play a role in regulating muscle repair. MyoD mRNA levels are also reported to be elevated in aging skeletal muscle.

One of the main actions of MyoD is to remove cells from the cell cycle (halt proliferation) by enhancing the transcription of p21. MyoD is inhibited by cyclin dependent kinases (CDKs). CDKs are in turn inhibited by p21. Thus MyoD enhances its own activity in the cell.