COL2A1 and Osteochondroma: Importantly, the number of R26ZSG reporter-positive cells increased substantially in SHP2 mutants, forming osteochondromas and enchondromas (Fig. 6a–c) that express Sox9, Col2a1, and Acan (Figs 6b and S8), similar to those that develop in mice lacking SHP2 in Col2a1-expressing cells.60,61 To investigate the cellular origins of these tumors, we examined the temporal expression of Bglap in cartilage as a readout of Bglap promoter activity.