IDH1 and chondrosarcoma: We observed a different grade progression over time (at relapse) in the 2 groups (IDH1/2mut or wt): IDHmut chondrosarcoma had a higher rate of grade progression at relapse, as compared to IDHwt. In particular 25 patients (21 cases presenting with G2 at diagnosis and 4 presenting with G3) had a new histologic evaluation at recurrence/progression: 5/9 (55%) of G2 IDH1/2mut tumors became higher grade at recurrence (1 became G3 and 4 dedifferentiated); while only 3/12 (25%) of G2 IDH1/2wt acquired a higher grade in the recurrence (3 became dedifferentiated).