HSPA5 and osteogenesis imperfecta: Upregulation of endoplasmic reticulum chaperone BiP (HSPA5), a key chaperone involved in regulation of the UPR, has initially been described in primary fibroblasts obtained from OI patients carrying proα1(I) and proα2(I) C-propeptide sequence variants but is absent in cells with glycine substitutions in the triple-helical regions (Chessler and Byers, 1993; Lamandé et al., 1995).