**2. TNBC vs. Basal**

TNBC is labeled based on IHC negativity of ER/PR/Her-2neu on tumor cells. However, based on gene expression profiles established through the 50-gene Prediction Analysis of Microarray (PAM50) assay, four "intrinsic subtypes" are defined: Luminal A, Luminal B, Basal-like, and Her-2-neu enriched. TNBC represents approximately 15–20% of all patients with breast cancer and shares various similarities with basal-like cancer [4]:


The basal-like subtype of breast cancer is characterized by a gene expression profile similar to that of the basal-myoepithelial layer of the normal breast; cytokeratins (5/6, 14, and 17), P-cadherin, EGFR17, and EGFR gene amplification (rarely) [5]. TP53 gene mutations are observed in up to 85% of cases [5, 6]. However, basal-like breast cancers, unlike "basal"/myoepithelial cells of normal breast, uniformly express cytokeratins 8 and/or 18 [5]. This questions microarray-based taxonomy of breast cancers that suggested that basal-like cancers would arise from basal/ myoepithelial cells. This has been answered in a recent study with the possibility that a subgroup of basal-like breast cancers may originate from luminal progenitors rather than basal myoepithelial cells of the breast [7].

Although TNBC and basal-like share many similarities and the terms are very often used interchangeably, they are different. Not all basal-like cancers lack ER, PR, and HER2, and not all TNBCs show a basal-like phenotype by expression array analysis (25% discordance) (**Figure 1**) [8].

Reasons for this discordance can be:


*Inter-Relationship of Ki-67 and Triple-Negative Breast Cancer DOI: http://dx.doi.org/10.5772/intechopen.109586*

**Figure 1.**

*Concordance between TNBC (IHC BASED) AND BASAL LIKE (genetic array analysis.*

4.Multigene expression data using hundreds of genes better capture the accurate biological profile compared with three or four individual surrogate biomarkers used to label TNBC [6]
