Mosaic animals, which contain somatic cells with different genotypes, allow researchers to track how mutation or deletion of target genes affects cell development. By knocking out a gene in a specific cluster of cells at a desired time, scientists can study its role without affecting the rest of the animal. Mutant genes in Drosophila mosaics can be visualized via fluorescent markers, but to date it's been difficult to develop a similar labeling system for mosaic vertebrates. Now, Liqun Luo and colleagues at Stanford University have helped solve this problem in mice with a new method called MADM – mosaic analysis with double markers.