While in osteochondroma, EXT1 or EXT2 are biallelically inactivated in at least a subset of the tumor cells [87], the cartilaginous cap of secondary peripheral ACT/CS1, in tumor progression, becomes gradually populated by cells with at least one functional copy of EXT1 or EXT2 [88], so that EXT-mutant alleles and EXT-wildtype alleles coexist [74]. This evidence concerns the gene EXT1 and Osteochondroma.