The most susceptible lineages were fetal myogenic progenitors and postnatal satellite cells but not Myf5- or Myf6-expressing postnatal committed myogenic progenitors: Of 18 Myf5CreER,Pax3:Foxo1,p53 mice given tamoxifen at 30 d of life (P30 [postnatal day 30]) and observed 43–510 d (median, 395), only one developed at age 401 d a tumor that was diagnosed as aRMS solid variant of the cranial muscle (Supplemental Fig. S1B–D). The gene discussed is MYF5; the disease is neoplasm.