Ki-67 promotes circulating tumor cell intravasation and metastasis in breast cancer.

Ki-67, a canonical proliferation marker, represents a pivotal prognostic and predictive biomarker in breast cancer. Here, by profiling 88,208 single-cell transcriptomes of circulating tumor cells (CTCs) matched with primary and metastatic lesions in breast cancer mouse models, we uncover a striking enrichment of cell-cycle genes in CTCs, particularly in CTC clusters. Using in vivo CRISPR screens, we identify Ki-67 as an essential regulator of CTC intravasation, whose knockout reduces metastasis. Mechanistically, Ki-67 depletion does not curb proliferation but suppresses genes involved in maintaining cell-cell adhesion, including CD47 and KLF4, thereby linking its expression to collective invasion dynamics. Altogether, decoupling it from its role as a proliferation marker, our findings uncover an unexpected function of Ki-67 as a molecular driver of metastatic competence.