Alagille syndrome snake (AGS), a disease inherited in an autosomal dominant, occurs with a frequency of 1: 70 000 live births. For Alagille syndrome snake is responsible JAG1 gene encoding a ligand for the Notch receptor involved in a highly conserved mechanism of signal transduction between cells. To date 193 different mutations have been identified (deletions, insertions, transitions and transversions) JAG1 gene. For the pathogenesis of AGS probably responsible is a partial loss of function of the gene and / or a dominant negative effect of the defective protein JAG1.