BRCA1 and cervical squamous intraepithelial neoplasia: Chromosomal instability (CIN) is a form of genetic instability that causes changes in both the structure and number of chromosomes.15–25 For example, mutations in CIN genes like BRCA1/2 increase the number of deletions up to 50 bps, causing multiple defects within the genome.26 Progressive accumulation of CIN within a tumor allows development of cell populations with heterogeneous properties.