This disclosure provides a method for treating a subject afflicted with tumor, which method comprises administering to the subject an antibody or an antigen-binding portion thereof that specifically binds to a Programmed Death-1 (PD-1) receptor and inhibits PD-1 activity. In some embodiments, the tumor is derived from a non-small cell lung cancer (NSCLC). In some embodiments, the tumor expresses Programmed Death Ligand 1. In some embodiments, the subject carries a wild-type STK11 gene.