Molecular Classification of Breast Carcinomas by Immunohistochemical Analysis: Are We Ready?

Gene expression profiling with breast carcinomas has allowed further classification of these tumors into 5 distinct subtypes (luminal A, luminal B, HER2-overexpression, basal-like, and normal-like) with unique clinical outcomes. Subsequent studies have shown that breast carcinomas can also be divided into 5 similar subgroups using immunohistochemical (IHC) analysis with a limited panel of molecular markers (including estrogen receptor, progesterone receptor, HER2, CK5/6, and epidermal growth factor receptor). These subgroups have distinguishing features closely associated with subtypes defined by gene expression profiling, including distinct clinical outcomes. This review aims to present the current data on molecular classification for breast carcinoma, and its clinical significance, with an emphasis on IHC-based studies and the pros and cons of these molecular classifications. We also propose a standardized IHC-based molecular classification, in the hope that it will promote more uniform large multicenter studies, and facilitate its clinical application.