Microvillus inclusion disease (DIAR12) is a congenital enteropathy characterized by neonatal-onset intractable secretory diarrhea, resulting in severe dehydration and metabolic acidosis. Patients may tolerate limited enteral feeding, but are dependent on total parenteral nutrition (TPN) and require eventual small bowel and/or liver transplantation. Pathologic hallmarks include variable loss of brush-border microvilli, microvillus inclusions, and accumulation of subapical vesicles in villus enterocytes (summary by Wiegerinck et al., 2014). Another form of microvillus inclusion disease, MVID1 (DIAR2; 251850), is caused by mutation in the MYO5B gene (606540). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700). Mutations in the STX3 gene that affect only isoform A (STX3A) cause DIAR12, whereas mutations in STX3 affecting both STX3A and isoform B (STX3B), which predominates in retinal tissue, cause a syndrome involving severe early-onset retinal dystrophy and MVID (RDMVID; 619446).