This document provides methods and materials involved in assessing samples (e.g., cancer cells) for the presence of a loss of heterozygosity (LOH) signature. For example, methods and materials for determining whether or not a cell (e.g., a cancer cell) contains an LOH signature are provided. Materials and methods for identifying cells (e.g., cancer cells) having a deficiency in homology directed repair (HDR) as well as materials and methods for identifying cancer patients likely to respond to a particular cancer treatment regimen also are provided.