Alexander Schier simply wanted to make sure he destroyed a gene in zebrafish embryos. So like many biologists these days, he turned to the genome-editing system known as CRISPR. But Schier, a developmental biologist at Harvard University, ended up doing much more than knocking out a gene. He and colleagues devised a new way to mark and trace cells in a developing animal. In its first test, described online today in Science, the researchers used CRISPR-induced mutations to reveal a surprise: Many tissues and organs in adult zebrafish form from just a few embryonic cells.