SLC35B1 and heart failure: Persistent ER stress–induced heart failure in AXER-KO medaka was well rescued by the constitutive expression of XBP1(S) but not of ATF6α(N) (Figs 6 and 7), allowing AXER-KO XBP1 SC/+ medaka to live 3 d longer after hatching (Fig 7D).