Alagille syndrome (MIM 118450) is an autosomal dominant disorder characterized by paucity of intrahepatic bile ducts, chronic cholestasis, pulmonary stenosis, butterfly-like vertebrae, posterior embryotoxon, and dysmorphic facial features. Most cases are caused by JAG1 gene mutations. We report the case of a 2-year-old Mexican mestizo patient with Alagille syndrome, having exhibited jaundice and cholestatic syndrome as of three weeks of age. Sequencing analysis of the JAG1 gene revealed the novel heterozygous mutation c.91dupG that originates a truncated protein and therefore a possibly diminished activation of the Notch signaling pathway. The latter may explain the severe phenotype of the patient. Since the mutation was not identified in the parents, it was considered a de novo event, highlighting the importance of molecular diagnosis and genetic counseling. In conclusion, this report widens the spectrum of JAG1 gene mutations associated with Alagille syndrome.