The etiology of Alagille syndrome is genetic heterogenous, about 2/3 cases are due to haploinsuficiency of JAG1 gene either by interstitial microdeletion in region 20p11.23 or by dominant point mutation. Clinical features are intrahepatic ductular hypo/aplasie, valvular or peripheral pulmonary stenosis, skeletal anomalies (hemivertebra, „butterfly“ vertebrae), typical long or triangular facies with long nose, anomalies of the eye (posterior embryotoxon and retinal pigmentary changes) and poor school performance. Phenotype is variable and severity varies widely and reflects the position and the extension of microdeletion or type of mutation. Incidence of syndrome is about 1:20 000 - 25 000 newborn children while some mild forms can be underdiagnosed. Differentiation between microdeletion and gene mutation is essential for estimation of genetic prognosis of reproduction in the family. A girl with Alagille syndrome due to gene mutation is refered and in her father a mosaic form could have not been excluded.

        Key words: Alagille syndrome, intersticial microdeletion of region 20p11.23, mutation in JAG1 gene, intrahepatic bilial ducts hypo/aplasia,