CRISPR/Cas systems are widespread, mobile systems for host defense against invasive elements such as phage. In these systems, Cas3 designates one of the core proteins shared widely by multiple types of CRISPR/Cas system. This model represents an HD-like endonuclease that occurs either separately or as the N-terminal region of Cas3, the helicase-containing CRISPR-associated protein.
Jiyao W et al.(2023). "The conserved domain database in 2023.",