The URLs for data presented herein are as follows: Cystic Fibrosis Mutation Database, http://www.genet.sickkids.on.ca/cftr/app Entrez, http://www.ncbi.nlm.nih.gov/entrez/ GeneTests, http://www.genetests.org/ HUGO, http://www.gene.ucl.ac.uk/nomenclature/ Human Gene Mutation Database, http://www.hgmd.cf.ac.uk Human Genome Variation Society, http://www.genomic.unimelb.edu.au/mdi/dblist/glsdb.html Mouse Genome Informatics, http://www.informatics.jax.org/ (for the Mouse Genome Database) Online Mendelian Inheritance in Animals (OMIA), http://omia.angis.org.au/ Online Mendelian Inheritance in Man (OMIM), http://www.ncbi.nlm.nih.gov/Omim/ PAHdb, http://www.pahdb.mcgill.ca/