XBP1 and ovarian hyperstimulation syndrome: Compared with patients without OHSS, follicles from OHSS patients express higher levels of XBP1 s in cumulus cells, which partly mediates upregulation of VEGFA, a key molecule in angiogenesis in the ovary that is regarded as a major cause of OHSS induction (Kosaka et al., 2007; Takahashi et al., 2016).