Ki-67 (MKI67) protein is a marker for proliferation commonly used in breast cancer pathology. It can be used to determine the growth fraction of cell populations in many tumors and tissue types. In the brain, it can be used to estimate rates of neurogenesis and aging-related neurogenerative decline. In breast cancer, the degree of expression of Ki-67 is used in conjunction with ER, PR and HER2 expression levels to subtype breast cancers. Higher levels of this protein are correlated with increased rates of proliferation (Zaha, 2014), absence of ER and PR and poor prognosis, and staining for Ki-67 in immunohistochemistry is thus important for determining the necessary treatment (Zorka, 2014). In normal tissues, Ki-67 has positive nuclear staining throughout the body, with strongest staining in germinal center cells and in subsets of cells in seminiferous ducts, squamous epithelia and the gastrointestinal tract. References: World Journal of Clinical Oncology. 2014. 5.3; 382-392, PMID: 25114853; Oncology 2014 Sep; 8; 107-111, PMID: 25249766; Aging Cell. 2017 Oct;16(5):1195-1199, PMID: 28766905; Int J Cancer 1983;31:13, PMID: 6339421;