Provided herein are methods of identifying a subject having a poly-ADP ribose polymerase (PARP) inhibitor-sensitive tumor by detecting a genomic gain in chromosome 1q21 and/or chromosome 20q13.3 in a tumor sample from the subject. Also provided are methods of identifying a subject having a PARP inhibitor-sensitive tumor by detecting gene amplification of a CHDIL gene or an RTEL1 gene in a tumor sample from the subject. Further provided are methods of treating a tumor with a genomic gain in chromosome 1q21 and/or chromosome 20q13.3 in a subject by administering an effective dose of a PARP inhibitor.