Researchers have discovered a likely reason why some tumor cells are inherently resistant — or become resistant over time — to the breast cancer drug Herceptin. Led by Colleen Sweeney, the team found that activating the MET and HER2 genes together substantially increased tumor-cell proliferation. They went on to show that inhibiting or depleting MET makes HER2-positive breast cancer cells more susceptible to Herceptin, while activating MET reduces the drug's effectiveness.