Activation of p21(CIP1/WAF1) in mammary epithelium accelerates mammary tumorigenesis and promotes lung metastasis

► Akt-activated p21 was primarily expressed in cytoplasm of cells in transgenic mice. ► Akt-activated p21 accelerated tumor onset in MMTV/neu mice. ► Akt-activated p21 promoted lung metastasis in MMTV/neu mice. ► In vivo evidence that cytoplasmic p21 functions as an oncogene.