Another factor contributing to CIN subtype formation is cell-cycle checkpoint defects caused by the upregulation of expression levels of mitotic checkpoint catalytic subunit 1 (CDK1) and its regulatory factors, cyclin B1/B2, cyclin A, cell division cycle 25 (CDC25), cyclin-dependent kinases regulatory subunit 1 (CKS1), and CKS2. This evidence concerns the gene CKS1B and cervical squamous intraepithelial neoplasia.