Alagille syndrome (AGS, MIM 118450) is associated with human chromosome band 20p12. To study this region, we constructed a 3.7-Mb physical map using 36 YACs isolated from the CEPH YAC library with three sequence-tagged sites (STS): D20S503, D20S41, and D20S188. New STSs were obtained from 6 isolated YAC end-fragments. Eighteen markers were ordered on the contig as follows:20ptel-D20S177-D20S175-D20S509- D20S5/D20S503-D20S506-D20S162-D20S504- D20S505-D20S507-D20S188-(D20S6-D20S27- D20S189)-D20S186-D20S41-D20S61-D20S492- D20S508-20pcen. A restriction map with the enzymes AscI, MluI, NotI, SacII, and SfiI was generated, revealing seven putative CpG islands. We established a YAC contig that spans the AGS region and thus will be valuable for cloning candidate genes and searching for DNA polymorphisms segregating with this syndrome.