COMP and pseudoachondroplasia: A similar upregulation of XBP1 signalling was seen in the Col10a1 N617K model of metaphyseal chondrodysplasia type Schmid (MCDS), but not in an allelic series of pseudoachondroplasia (PSACH) causing Comp mutations, indicating gene product specificity of this arm of the UPR [6, 31, 32].