An early clinical trial of the drug MK-3475 in non-small cell lung cancer (NSCLC) has yielded promising results. MK-3475 targets PD-1, a protein on the surface of immune cells. Another protein, PD-L1, is present on many tumor cells and can bind to PD-1, which deactivates immune cells. MK-3475 blocks PD-1, allowing the immune cells to keep attacking cancer cells. Patients with advanced NSCLC who had failed at least two other treatments were given MK-3475. Tumors shrank in 24% of the patients overall. However, tumor shrinkage occurred in 67% of patients with high levels of PD-L1 on their tumors, compared to only 9% of others. PD-L1 levels may therefore help predict which patients will likely respond to MK-3475.