A protein derived from the CRISPR-Cas bacterial immune system that has been co-opted for genome engineering. Cas3 is a standalone nuclease-helicase that is recruited to targets by a separate, crRNA-guided complex called Cascade. Once Cascade identifies a complementary DNA target, it recruits Cas3 to move along the DNA, cutting as it goes. In CRISPR immunity, cutting phage DNA prevents phages from replicating and destroying the host cell. When used for genetic engineering, this approach can be used to delete thousands of base pairs around the target site.