STX3 and microvillus inclusion disease: Mutations in STX3 or STXBP2 cause MVID and result in severe diarrhea (Stepensky et al., 2013; Vogel et al., 2017b; Wiegerinck et al., 2014), indicating that, in addition to MYO5B-regulated trafficking, the membrane fusion machinery also has an important role in the maintenance of IEC polarity (Fig. 2).