From the August 2001 to the Nov 2002 freeze, this track was based on tiling path (TPF) maps curated by the sequencing centers responsible for each chromosome, which were integrated into an assembly done by NCBI. Beginning with the April 2003 freeze, the chromosome coordinators at the individual sequencing centers took over complete responsibility for preparing the assembly of their choromosomes in AGP format. The files provided by these centers are checked and validated at NCBI, and form the basis for the definition of the physical map contigs.