Schematic overview of the steps involved in adaptive immunity as provided by CRISPR–Cas. (A) Adaptation stage, featuring the introduction of viral DNA/RNA (curved gray line) with protospacer 1 (dark blue) and an adjacent PAM (red). Protospacer 1 is incorporated in the bacterial genome as spacer 1 (dark blue) flanked by two repeats (orange) in the CRISPR array alongside spacer 2 (green). (B) Expression stage, wherein a Cas protein (light blue) is produced and pre-crRNA is transcribed and processed into mature crRNAs, which subsequently form functional RNP complexes with the Cas protein. (C) Interference stage, where a second infection is targeted for degradation by crRNA-guided Cas proteins, binding to matching protospacers adjacent to PAMs.