These results, together with previous studies showing that multipotent stem cells must commit to unipotent progenitors in order for Ptch1 deficiency to promote growth of medulloblastoma (Schüller et al., 2008; Yang et al., 2008b), suggest that enchondroma and osteosarcoma are originated from the early chondrocyte and osteoblast progenitors derived from the Prrx1+ MSCs. This evidence concerns the gene PTCH1 and osteosarcoma.