For example, mice carrying a tag or a fluorescent reporter construct in Nanog, Sox2 and Oct4 (important stem cell genes) as well as a Mecp2 conditional mutation (a Rett syndrome gene) were generated by one-step co-injection of zygotes with Cas9 mRNA, different sgRNAs and DNA vectors (Fig. 3a) [18]. Here, MECP2 is linked to Rett syndrome.