This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Molecular classification of breast cancer is based on - 
Select and explain. 
A. Gene expression profiling, B. Expression of hormone receptors (ER/PR), C. Serum hormone levels, D. In-vitro response to chemotherapeutic agents
A: Gene expression profiling
Answer: A: Gene expression profiling 
Explanation: . Gene expression profiling