Toward the mammalian transcript map

The advances which made positional cloning possible are now a decade old. Positional cloning has become easier over this period because resources have improved. This started with shared gridded large-insert genomic libraries, has continued with gridded cDNA libraries, Expressed Sequence Tag (EST) sequencing and mapping, and is moving into later stages of gene and gene product characterisation and of course genome sequencing. It is clear that it must get easier still because few Quantitative Trait Loci (QTL) have yet been cloned. With the large regions involved in QTL hunts, the key resource is a comprehensive, richly annotated transcript map as a source of candidate genes. Since some tissues or developmental stages are more easily studied in particular species, the best transcript map will be a composite of mammalian maps. In mouse our priority is to make a high quality physical map. We are making a well anchored second-generation Yeast Artificial Chromosome (YAC) map, from which a sequence-ready Bacterial Artificial Chromosome (BAC) map can be derived in parallel with transcript mapping.