Our data support the concept that (1) MVID results from the loss of MYO5B function in enterocytes, (2) that a complete lack of protein and greatly truncated MYO5B proteins do not cause primary liver disease, (3) that solitary primary cholestatic liver disease results from the expression of mutant MYO5B proteins that cause aberrant protein–protein interactions in hepatocytes, (4) and that expressed non-functional MYO5B protein causes both intestinal and hepatic disease. The gene discussed is MYO5B; the disease is microvillus inclusion disease.