Hi Julie, C2C12 is a tricky cell line, you can find yet several differences among different C2C12 clones from various brand (ATCC, sigma, etc) some clones expressing Pax7 some others not. Moreover often growing C2C12 at very low confluence you could favour proliferating cpability versus differetiantin one. Therefore I suggest to try with a high number of clones and overall test straight myotube formation capacity. regarding cell growth C2C12 are quite quick to grow, but again it dipends form the original clone you have.