Mapping of an informative Hcrtr2 gene crossover observed in a litter of Dachshund with a recessive Hcrtr2 mutation causing narcolepsy. Six microsatellite markers located in the vicinity ofHcrtr2 were used (Lin et al. 1999, and Fig. 1A). Alleles are arbitrarily numbered by size and displayed in parenthesis. Five sequenced variants (four single nucleotides polymorphisms and a 3-bp deletion in exon 1) described in the Results section were also used. Note that the informative crossover occurred between marker 6-28-2 and exon 5 (Fig. 1A). The disease-causing mutation is the third polymorphism indicated in bold (exon 1, G/A).