The analysis of genetically mosaic organisms, which carry subsets of cells that are mutant for one or more genes, can reveal important information about a gene's role in development. In the first of a series of educational Primer articles on this topic, Yochem and Herman (see ) describe how genetic mosaics are generated, identified and analysed in the nematode Caenorhabditis elegans. As they discuss, genetic mosaics can provide a way around the problem of embryonic lethality that is caused by some genetic mutations. They also allow researchers to ask key questions about how a gene functions in development, such as whether a gene acts cell autonomously (when its loss effects only the cells that express it) or non-cell autonomously(when its loss affects other, sometimes wild-type, cells). Genetic mosaic approaches to testing a gene's function have thus shed important light on developmental mechanisms in this and other organisms.
