COMP and chondrodysplasia: We have previously shown that a reduction in chondrocyte proliferation is a key mechanistic finding in mouse models of chondrodysplasia caused by mutations in Matn3 (V194D ↓16%; Leighton et al. 2007) and COMP (T585M ↓24% (Pirog-Garcia et al. 2007) and 469Del ↓17% (Suleman et al. 2011)) and confirmed that reduced proliferation is the major driver of reduced long bone growth in mouse ER stress phenocopies (ColIITgrdw ↓21% (Gualeni et al. 2013) and ColIITgcog ↓12% (Rajpar et al. 2009)).