Cardiac Glycosides (CGs) are commonly used to treat congestive heart failure. CGs inhibit the Sodium Potassium ATPase (Na+/K+ ATPase) pump. Interestingly, CGs have been suggested to inhibit proliferation and migration of breast cancer cells. A pool of non-pumping Na+/K+ ATPase reportedly localizes in specific membrane organelles, caveolae, by interacting with the structural protein caveolin-1. It has been postulated that Na+/K+ ATPase forms a complex with caveolin-1 and the tyrosine kinase Src, and that binding of CGs to Na+/K+ ATPase activates Src-dependent signalling cascades. In this project we explored whether CGs reduce proliferation in breast cancer cells and whether these effects might involve Src and ERK (Extracellular-signal-Regulated Kinases).