KI-67, also known as Ki-67 or MKI67, is a protein encoded by the MKI67 gene in humans. Ki-67 is a 350-400kDa nuclear protein and a member of mitosis related proteins of chromosomes. In addition, Ki-67 is involved in ribosomal RNA transcription, and its inactivation inhibits ribosomal RNA synthesis. Ki-67 interacts with 160kDa Hklp2, which promotes centrosome separation and spindle bipolarity. It also interacts directly with NIFK and binds to UBF, thus playing a role in the synthesis of ribosomal RNA. Among patients with ER-positive breast cancer, Ki-67 represents a highly proliferative subgroup, allowing them to benefit more from adjuvant chemotherapy.