Key points
1.	Spontaneous breast cancer regression is a rare biological phenomenon.
2.	Its mechanism has yet to be clearly revealed.
3.	It is predicted that the most likely hypothesis may be trauma and infection.
4.	The earliest regression period after biopsy is thought to be 21 days.
5.	Patients with clinical regression after biopsy should be evaluated in detail.
6.	Molecular analysis in living tissue samples may contribute to revealing the mechanism.