 CRISPR-slash-cas9 is a versatile genome editing technology that is widely used for studying the functionality of genetic elements. However, the high frequency of off-target activity, greater than or equal to 50%, is one major concern, especially for therapeutic and clinical applications. Here, we review the basic mechanisms and methods for detecting off-target mutations. This article is authored by Xiao Huijing, Lewis Y.T., Xiao Gang Wang, and others.