Irrespective of EXT1 pathogenic variant or no EXT1/EXT2 pathogenic variant identified, the histopathology classification of anaplasia of their tumors was grade I. The two patients who presented loss‐of‐function mutations in exon 8 of EXT2 gene underwent malignant degeneration to chondrosarcoma grade II, which may suggest that mutations in EXT2 gene may be responsible for a propensity of MO patients to develop tumors that are more aggressive. This evidence concerns the gene EXT1 and chondrosarcoma.