We report on the progress of a project funded by the Wellcome Trust to produce over 100 recombinant inbred mouse lines as part of the Collaborative Cross (CC) genetic reference panel. These new strains of mice are being derived from a set of eight genetically diverse founders. The genomes of the finished strains will be mosaics of the founder strains' genomes with a high density of independent recombination breakpoints. The CC mice will be available for distribution free of any intellectual property constraints to serve as a community resource for systems genetics studies.