Tumors with high levels of microsatellite instability (MSI-H), such as some subsets of colorectal, endometrial and gastric cancers, are associated with a hypermutated phenotype and a specific dependency on the WRN gene. Pharmacologic inhibition of WRN has been shown to selectively impair the viability of MSI-H cancer cells, effectively blocking tumor growth while sparing microsatellite-stable (MSS) cells.