**Abstract**

Triple-negative breast cancer (TNBC) is a heterogeneous group characterized by an early onset, aggressive course of the disease, a higher tendency of visceral metastases, and a poorer prognosis. It is also associated with basal-like phenotype and germline mutations for BRCA genes in 10–20% and somatic mutations in 3–5% of cases. Based on gene expression profiling, TNBC is divided into four tumor-specific subtypes (Basal-like 1, Basal-like 2, Mesenchymal, and Luminal androgen receptor) with different clinical, prognostic, and therapeutic implications. The Ki-67 antigen, a non-histone nuclear protein, is a surrogate marker to assess tumor proliferation. As TNBCs are expected to be highly proliferating tumors, a higher baseline Ki-67 level has been seen. Although a higher Ki-67 level is associated with a higher pathological complete response rate, the best cutoff point of this marker as a prognostic and predictive factor in TNBC remains unclear.

**Keywords:** triple negative, breast cancer, Ki 67 expression, chemotherapy response, prognosis, predictive marker, survival, quality of life
