Abstract
Progressive familial intrahepatic cholestasis is a clinical description of a phenotype, which we now realize has several different genetic aetiologies. The identification of the underlying genetic defects has helped to elucidate important aspects of liver physiology. The latest addition to this family of diseases is tight junction protein 2 (TJP2) deficiency. This protein is also known as zona occludens 2 (ZO-2). The patients, so far presented, all have homozygous, protein-truncating mutations. A complete absence of this protein was demonstrated. These children presented with severe liver disease, some manifesting extrahepatic features. By contrast, embryonic-lethality was seen in ZO-2 knockout mice. This discovery highlights important differences, not just between species, but also between different epithelia in humans. This commentary discusses the recently presented findings, and some of the issues that arise.