Objective · To perform JAG1 gene screening and mutation function analysis for an Alagille syndrome patient. Methods · Clinical data and auxiliary examination results of the patient and his parents were collected. Genomic DNA was extracted for JAG1 mutation screening. Vectors with wild-type and mutant JAG1 expressions were constructed and transfected into NIH-3T3 cells. mRNA and protein expression levels were analyzed with real-time RT-PCR and Western blotting. Endoglycosidase H digestion experiment was performed to analyze the glycosylational structure. The effect of JAG1 protein on activating the transcription factor RBP-Jκ in Notch signaling pathway was detected with RBP-Jκ luciferase reporter gene assay. Results · A novel missense mutation c.1655C>T (p.Pro552Leu) was detected in the patient and his father, but was not found in his mother or healthy controls. Wild-type and mutant JAG1 had no difference in mRNA or protein expression levels. Post-translational glycosylational structure of the mutant JAG1 was the same as wild-type JAG1. However, the effect of activating the transcription factor RBP-Jκ in Notch signaling pathway was reduced in mutant JAG1 than in wild-type JAG1. Conclusion · The JAG1 missense mutation carried by the patient results in impaired function of JAG1, which may be the cause of Alagille syndrome.