Alagille syndrome (AGS) has been assigned to 20p11.23-20p12.2 according to minimum overlap between deletions observed on the chromosome 20 short arm of 9 patients. We report here the localisation of 5 microsatellite markers (D20S41, D20S48, D20S50, D20S56, and D20S58) within the deletion of one AGS patient. This study allows an estimation of the genetic extent of this deletion as being between 30 and 36 cM, and demonstrates its paternal origin. The search for submicroscopic deletions in 23 AGS patients, by typing these 5 markers, failed to reveal allelic loss. However, these results lead to the proposition that the AGS locus lies in one of the seven intervals defined by the six microsatellite markers in the region flanked by D20S5 and D20S18.