This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Breast, Subject name: Surgery
Molecular classification of breast cancer is based on: 
Select and explain. 
A. Serum hormone levels, B. Expression of hormone receptors (ER/PR), C. In-vitro response to chemotherapeutic agents, D. Gene expression profiling
D: Gene expression profiling
Answer: D: Gene expression profiling 
Explanation: Molecular classification of breast cancer is based on gene expression profiling. "Gene expression profiling, which can measure the relative quantities of mRNA for essentially every gene, has identified five major patterns of gene expression in the NST group: luminal A, luminal B, normal, basal-like, HER2 positive. These molecular classes correlate with prognosis and response to therapy, and thus have taken on clinical impoance".