ATF6 and Schmid metaphyseal chondrodysplasia: Nevertheless, the ablation of Atf6α resulted in a further 14% expansion of the HZ of MCDS mice (Fig. 2a & b; p < 0.01) which is indicative of more severe ER stress in the Atf6α−/−/MCDS mouse since the degree of HZ expansion in MCDS directly and positively correlates with the disease severity [7,8,28].