Alagille syndrome (AGS) is an autosomal dominant disorder caused by mutations in the JAG1 gene. The study identifies JAG1 as the gene responsible for AGS, which is associated with intrahepatic cholestasis, heart, eye, and vertebral abnormalities, as well as a characteristic facial appearance. Researchers mapped the gene to 20p12 through cytogenetic deletions and narrowed the critical region to 250 kb using fluorescent in situ hybridization (FISH) and submicroscopic deletions in two AGS patients. The JAG1 gene encodes a ligand for the Notch receptor, which is critical for cell fate determination during development, making it a strong candidate for AGS. The study determined the complete exon-intron structure of JAG1 and identified various mutations, including frame-shift mutations, splice donor mutations, and a mutation that abolishes RNA expression. These mutations lead to haploinsufficiency of JAG1, resulting in AGS. The gene was found to be located within the 250-kb critical interval, and its expression was widely observed in various tissues, with the highest levels in the ovary, prostate, pancreas, placenta, and heart. The study also analyzed the genomic structure of JAG1, revealing 26 exons and 25 introns, with intron sizes ranging from 89 bp to nearly 9 kb. A polymorphic marker, D20S1154, was identified as a useful tool for detecting submicroscopic deletions. Mutation analysis of AGS patients revealed several mutations, including a frame-shift mutation in exon 22, an insertion in exon 22, and a splice donor mutation in exon 23. These mutations were associated with the characteristic features of AGS. The study concludes that AGS is caused by haploinsufficiency of JAG1, as mutations in the gene lead to the loss of function, resulting in the clinical features of the syndrome. The findings highlight the importance of JAG1 in developmental processes and its role in the pathogenesis of AGS. The study also emphasizes the need for further research to understand the genotype-phenotype correlations in AGS and to explore the broader implications of Notch signaling in human development.Alagille syndrome (AGS) is an autosomal dominant disorder caused by mutations in the JAG1 gene. The study identifies JAG1 as the gene responsible for AGS, which is associated with intrahepatic cholestasis, heart, eye, and vertebral abnormalities, as well as a characteristic facial appearance. Researchers mapped the gene to 20p12 through cytogenetic deletions and narrowed the critical region to 250 kb using fluorescent in situ hybridization (FISH) and submicroscopic deletions in two AGS patients. The JAG1 gene encodes a ligand for the Notch receptor, which is critical for cell fate determination during development, making it a strong candidate for AGS. The study determined the complete exon-intron structure of JAG1 and identified various mutations, including frame-shift mutations, splice donor mutations, and a mutation that abolishes RNA expression. These mutations lead to haploinsufficiency of JAG1, resulting in AGS. The gene was found to be located within the 250-kb critical interval, and its expression was widely observed in various tissues, with the highest levels in the ovary, prostate, pancreas, placenta, and heart. The study also analyzed the genomic structure of JAG1, revealing 26 exons and 25 introns, with intron sizes ranging from 89 bp to nearly 9 kb. A polymorphic marker, D20S1154, was identified as a useful tool for detecting submicroscopic deletions. Mutation analysis of AGS patients revealed several mutations, including a frame-shift mutation in exon 22, an insertion in exon 22, and a splice donor mutation in exon 23. These mutations were associated with the characteristic features of AGS. The study concludes that AGS is caused by haploinsufficiency of JAG1, as mutations in the gene lead to the loss of function, resulting in the clinical features of the syndrome. The findings highlight the importance of JAG1 in developmental processes and its role in the pathogenesis of AGS. The study also emphasizes the need for further research to understand the genotype-phenotype correlations in AGS and to explore the broader implications of Notch signaling in human development.