In HumanGAPP, almost 1.000 rat-human orthologous gene pairs were used as anchor points to create a framework from which the complete human-rat comparative map was built. At least two consecutive orthologous gene pairs on the same pair of a human and a rat chromosome were used to define an Evolutionary Conserved Segment (ECS). In all, 130 human/rat ECS were found. In contrast to the mouse-rat comparative map, this human-rat map is arranged with the human chromosome as a framework using the human base pair positions obtained from the UCSC Genome Browser (17) as a matrix. Thus each gene is ordered and defined by its start codon. Supporting data, such as gene symbol, gene description, OMIM accession ID, GDB accession ID (13) and LocusLink ID were downloaded from NCBI (8). The human framework data is linked to rat genome information from RatMap, and all rat and human genes within any ECS, including the homologous gene pairs, are accessible through a web based interface. For rat genes aligned with BACs (obtained from the RatMapped database), details on alignment and links to the corresponding rat BACs are presented.