Illustration of a Cas4/Cas1 complex (right) and a synthetic oligonucleotide (left) used to stabilise that complex. Cas1 and Cas4 are proteins associated with the CRISPR (Clustered Regularly Interspersed Palindromic Repeats) adaptive immune system in bacteria and archaea. When these organisms encounter viral DNA (deoxyribonucleic acid), the CRISPR system copies and inserts segments, or spacers, of the invading nucleic acid between sets of repeating DNA CRISPR sequences in its own genome. Cas1 and Cas4 are both endonucleases, enzymes that cut DNA strands, and are involved in the acquisition of the spacers.