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A revolution in cancer research is underway. Spurred by the advent of the CRISPR-Cas9 technology, new methods to probe the mammalian genomes are being developed. By providing simple, flexible, and cost-effective ways to edit and manipulate the genome of somatic cells of adult animals, these new methods present the opportunity to model cancer progression in vivowith an unprecedented degree of sophistication. Here we provide a brief overview of this exciting and fast-moving field.