The International Knockout Mouse Consortium (IKMC; http://www.mousephenotype.org ) has generated mutations in almost every protein-coding mouse gene and is completing the companion Cre driver resource to expand tissue-specific conditional mutagenesis. Accordingly, the IKMC has carried out high-throughput gene trapping and targeting producing conditional mutations in murine embryonic stem cells in more than 18,500 genes, from which at least 4900 mutant mouse lines have been established to date. This resource is currently being upgraded with more powerful tools, such as visualization and manipulation cassettes that can be easily introduced into IKMC alleles for multifaceted functional studies. In addition, we discuss how existing IKMC products can be used in combination with CRISPR technology to accelerate genome engineering projects. All information and materials from this extraordinary biological resource together with coordinated phenotyping efforts can be retrieved at www.mousephenotype.org . The comprehensive IKMC knockout resource in combination with an extensive set of modular gene cassettes will continue to enhance functional gene annotation in the future and solidify its impact on biomedical research.