PIK3CA and breast cancer: To test whether our vector system could efficiently introduce both Indel and substitution editing in mammary cells in rats, we attempted to Indel-edit Tp53 and substitution -edit Pik3ca, two genes that are known to be co-mutated in substantial proportions of human breast cancers (10.5%; METABRIC), and have been reported to collaborate in transforming human and mouse mammary cells18,19 and cause worse prognosis20.