Definition

An elevated per-division rate of chromosome missegregation resulting in ongoing changes to chromosome number in daughter cells.

Synthesis

Chromosomal instability is established across sources as an elevated rate of chromosome missegregation during cell division that generates ongoing changes in chromosome number, producing aneuploidy and cellular heterogeneity through mechanisms such as merotely, improper microtubule-kinetochore attachments that occur even in the absence of classic mitotic defects like spindle assembly checkpoint failure. A key mechanistic relationship links chromosomal instability to immune surveillance, where the genomic chaos generates immunogenic signals that recruit immune cell infiltration into tumors, representing a significant fitness cost that unstable cells must overcome through active immune evasion strategies, particularly via Stat1 signaling inactivation combined with Myc activation. The most contested aspect of chromosomal instability concerns its dual role in cancer, where it demonstrates paradoxical tumor-suppressive and oncogenic effects depending on context: elevated aneuploidy promotes spontaneous tumor formation in aged animals but simultaneously inhibits chemically or genetically induced tumorigenesis, revealing that the same genomic instability phenotype can either drive or prevent malignant transformation based on the cellular environment and initiating events.