Once the "master controller" MyoD has become active, SETDB1 is required to maintain MyoD expression within the cell. Setdb1 appears to be necessary to maintain both MyoD expression and also genes that are specific to muscle tissues because reduction of Setdb1 expression results in a severe delay of myoblast differentiation and determination. In Setdb1 depleted myoblasts that are treated with exogenous MyoD, myoblastic differentiation is successfully restored. In one model of Setdb1 action on MyoD, Setdb1 represses an inhibitor of MyoD. This unidentified inhibitor likely acts competitively against MyoD during typical cellular proliferation. Evidence for this model is that reduction of Setdb1 results in direct inhibition of myoblast differentiation which may be caused by the release of the unknown MyoD inhibitor.