The acquisition of metastatic ability by tumor cells has been considered a late event in the evolution of malignant tumors for decades. In this study, mouse mammary cells were engineered to express the inducible oncogenic transgenes MYC and KrasD12, or polyoma middle T, and introduced into the systemic circulation of a mouse. The cells were able to bypass transformation at the primary site and develop into metastatic pulmonary lesions upon immediate or delayed oncogene induction. Perspective at http://www.sciencemag.org/cgi/content/full/321/5897/1785.