High GGT values often reflect rare genetic cholangiopathies such as Alagille syndrome (OMIM #118450), Renal Cysts and Diabetes Syndrome (RCAD) (OMIM #137920), alpha-1-antitrypsin deficiency (OMIM #107400), ABCB4 disease (OMIM #171060, Phenotype MIM number 60234), and neonatal sclerosing cholangitis (OMIM #61739) (involving the CLDN1, DCDC2, KIF12, or PPM1F genes respectively). Here, CLDN1 is linked to renal cysts and diabetes syndrome.