Microsatellite instability (MSI) in TCGA qualified uterine carcinosarcoma was evaluated by the Biospecimen Core Resource at Nationwide Children’s Hospital (TCGA Center 23). MSI-Mono-Dinucleotide Assay was performed to test a panel of four mononucleotide repeat loci (polyadenine tracts BAT25, BAT26, BAT40, & transforming growth factor receptor type II) & three dinucleotide repeat loci (CA repeats in D2S123, D5S346, & D17S250). Two additional pentanucleotide loci (Penta D & Penta E) were included in this assay to evaluate sample identity. Multiplex fluorescent-labeled PCR & capillary electrophoresis was used to identify MSI if a variation in the number of microsatellite repeats was detected between tumor and matched non-neoplastic tissue or mononuclear blood cells. Equivocal or failed markers were re-evaluated by singleplex PCR. Tumor DNA was classified as microsatellite-stable (MSS) if zero markers were altered, low level MSI (MSI-L) if less than 40% of markers were altered and high level MSI (MSI-H) if greater than 40% of markers were altered. In the MSI-Mono-Dinucleotide Assay, this equated to MSI-L classification if one or two markers were altered, and MSI-H if three to seven markers were altered. Penta D and E markers were scored in the same manner as the MSI markers, however they did not contribute to MSI Class calculation.