FOXO1 and breast cancer: Herein, we demonstrate that: (1) elevated levels of Spy1 is a transforming event, (2) Spy1-mediated transformation relies on the activation of Cdk1 and may mediate an inhibition of the pro-apoptotic regulator FOXO1, (3) levels of Spy1 protein are highly elevated in aggressive human breast cancers and (4) downregulation of Spy1 can significantly inhibit breast cancer cell growth.