Abstract . Interactions between tumour cells and the surrounding microenvironment contribute to tumour progression, metastasis and recurrence 1 , 2 , 3 . Although mosaic analyses in Drosophila have advanced our understanding of such interactions 4 , 5 , it has been difficult to engineer parallel approaches in vertebrates. Here we present an oncogene-associated, multicolour reporter mouse model—the Red2Onco system—that allows differential tracing of mutant and wild-type cell...