Abstract: Methods for detecting a deficient DNA repair mechanism and/or evaluating microsatellite instability in a sample from a subject are described. The disclosed methods comprise analyzing nucleic acid sequence data for a plurality of microsatellite loci to calculate a microsatellite instability (MSI) score for the sample from a ratio of the number of microsatellite loci that exhibit variant alleles to the total number of microsatellite loci that meet, e.g., a minimum sequencing coverage requirement. The MSI score is compared a first threshold and, m some cases, to a second threshold to assign a status of high microsatellite instability?(MSI-H), microsatellite stable (MSS), or equivocal microsatellite instability (MSI-E) for the sample.