COMP and multiple epiphyseal dysplasia: The role of ER stress in GSDs might best be exemplified by mutations in COMP, matrilin-3 and type X collagen resulting in pseudoachondroplasia (PSACH), multiple epiphyseal dysplasia (MED) [8] and metaphyseal chondrodysplasia type Schmid (MCDS), respectively (Figure 1 & Table 1) [16].