Cis- and trans-cleavage activity of CRISPR-Cas9 and its new developments in diagnostics

CRISPR-Cas system is an adaptive immune system of prokaryotes against foreign invading viruses. Type II CRISPR-Cas9 system, as the earliest of the Class II systems to be discovered and applied to gene editing, had its new discovery of sharing trans-cleavage activity with other Class II systems not long ago. In this review, we summarize the molecular mechanism of cis- and trans-cleavage of target nucleic acids by the Class II type II CRISPR-Cas system, introduce the nucleic acid detection platforms developed based on its trans-cleavage activity, and compare them with those developed based on the type V, type VI, and type I CRISPR-Cas systems.