On one hand, assuming that all IDH-mutated tumors (as well as enchondromas) observed in OD-MS patients derive from one IDH-mutant cell giving rise to different lineages, the observation of different tumors arising in organs deriving from the neuroectoderm, mesoderm and endoderm points towards a very early post-zygotic event for the IDH mutation. The gene discussed is IDH1; the disease is myeloid sarcoma.