Genomic instability (GIN) is a hallmark of cancer and promotes tumorigenesis through the accumulation of DNA damage and chromosome missegregation. Micronuclei (MN), which arise from chromosome fragments or whole chromosomes excluded from the primary nucleus during mitosis, are a robust indicator of GIN and are frequently observed at elevated levels in the erythrocytes of mouse models with defective DNA damage response (DDR) pathways. However, the genomic features that predispose specific chromosomal regions to micronucleus formation, as well as how distinct DDR pathways influence this process, remain poorly understood. Furthermore, it is unclear how models exhibiting elevated GIN but impaired DDR signaling balance the accumulation of mutagenic events with mechanisms that suppress tumorigenesis. Indeed, our data indicate that defects in a key DDR mechanism involving the RAD9-RAD1-HUS1 (9-1-1) complex both elevate GIN but suppress tumor development, likely because cancer cells are dependent on the 9-1-1 complex and related mechanisms to tolerate the elevated DNA damage and replication stress they experience. These proposed studies will establish how checkpoint signaling and DNA repair pathways shape chromosome fragmentation, define the genomic determinants of micronucleus formation, and provide new insights into the mechanisms linking genome instability to cancer initiation and progression, with important implications for cancer therapies based on inhibition of DNA repair and other aspects of the DDR.