**2.4 Ki-67**

The Ki-67 antigen, a nonhistone nuclear protein, was identified by Scholzer and Gerdes in 1983 in a Hodgkin lymphoma cell line [19]. The Ki-67 antigen encodes two protein isoforms with 345 and 395 kDa molecular weights [20]. This protein is expressed in the G1, S, G2, and M phase of the cell cycle but is absent in resting cells (G0) [20, 21]. Therefore, the nuclear expression of Ki-67 can be evaluated to assess tumor proliferation by IHC. The Ki-67 protein has a half-life of only 1–1.5 hours. Therefore, the quantity of Ki-67 present at any time during the cell cycle is regulated by a precise balance between synthesis and degradation [20].

### **2.5 Is Ki-67 a prognostic or a predictive marker?**

A prognostic biomarker indicates the likely course of the disease in an untreated individual, and a predictive biomarker identifies subpopulations of patients most

likely to respond to a given therapy. An increased Ki-67 is linked to a worse prognosis and an increased response to neoadjuvant chemotherapy. As Ki-67 represents proliferating tumor, a high level will translate to an increased response. However, same is not true for prognoses. Increased Ki-67 is an adverse prognostic factor in HR-positive tumors, and patients with low Ki-67 tumors have the best prognosis [21]. Whereas, in HR-negative tumors, low proliferating tumors have the worst prognosis. This phenomenon is also reported by Cortazar et al. in their meta-analysis, which shows that increased pCR rates are linked to better survival in the HR-negative subgroup [22]. At the same time, chemotherapy response does not affect the prognosis in HR-positive tumor. Based on this, Denkert et al. reported the biological plausibility of three different groups of tumors [21]:

