Creating cell type-specific mutants by enhancer mutagenesis.

Cell signaling plays an essential role in development, and knowledge of the identities of the cells sending the signal is critical. This can be a challenge, since signaling pathways and ligands are commonly used at multiple times and in multiple cell types during development. One solution to this problem is to create cell type-specific mutants using CRISPR/Cas9 to mutate enhancers that control different patterns of expression. In this issue of , Rogers and colleagues (pp. 634-638) provide the first use of this method in to solve a long-standing issue in patterning of the embryonic central nervous system.