ROSA26 is a locus used for constitutive, ubiquitous gene expression in mice. It was first isolated in 1991 by the group of Philippe Soriano in a gene-trap mutagenesis screen of embryonic stem cells (ESCs). Over 800 knock-in lines have been created based on the ROSA26 locus according to the MGI database. The human homolog of the ROSA26 locus has been identified. ROSA stands for <u>R</u>everse <u>O</u>rientation <u>S</u>plice <u>A</u>cceptor, named after the lentivirus genetrap vector.