The disclosure provides a method for treating a subject afflicted with a tumor derived from a small cell lung cancer (SCLC) having a high tumor mutational burden (TMB) status comprising administering to the subject a monotherapy comprising an anti-PD-1 antibody or a combination therapy comprising an anti-PD-1 antibody and an anti-CTLA-4 antibody. The present disclosure also provides a method for identifying a subject suitable for treatment with an anti-PD-1 antibody or a combination therapy comprising an anti-PD-1 antibody and an anti-CTLA-4 antibody comprising measuring a TMB status of a biological sample of the subject. A high TMB status identifies the patient as suitable for treatment with an anti-PD-1 antibody or antigen-binding portion thereof. The TMB status can be determined by sequencing nucleic acids in the tumor and identifying a genomic alteration, e.g., a somatic nonsynonymous mutation, in the sequenced nucleic acids.