Emily Waltz, 14 April 2016, Nature News, http://www.nature.com/news/gene-edited-crispr-mushroom-escapes-us-regulation-1.19754
Researchers at Pennsylvania State University have used CRISPR/Cas9 to knock out one of six polyphenol oxidase genes in the common white mushroom. Polyphenol oxidases are a class of enzymes that cause browning in cut mushrooms. By knocking out one polyphenol oxidase gene researchers were able to decrease the total polyphenol oxidase activity by approximately 30%, prolonging the shelf life of the mushroom. Since this mushroom does not contain a transgene, the USDA has ruled that it does not fall under its regulations for genetically modified organisms.