Researchers discovered an unexpected twist in the tumor vs. T cell battle. Tumors employ a protein called PD-L1 to blind T cells from carrying out their functions. PD-L1 protects tumors through a ‘molecular brake’ known as PD-1. Researchers have found that some tumor cells display not only their PD-L1 weapon, but also the PD-1 ‘brake,’ essentially a neutralizing function. The unexpected mechanism that could help determine whether a cancer patient will respond to immunotherapy.