Cdkn1c drives muscle differentiation through a positive feedback loop with Myod

► Myod and Cdkn1c (p57Kip2) interact genetically to promote myogenesis. ► Cdkn1c expression is Hedgehog-dependent in zebrafish adaxial cells. ► Cdkn1c promotes Myod protein accumulation. ► MRFs maintain Cdkn1c expression in vivo, thereby creating a positive feedback loop. ► Cdkn1c is not required for adaxial cell cycle exit, but to drive differentiation.