Figure 2. Adaptation to a new invader. Upon entering the cell, the invading foreign nucleic acid is recognized by Cas proteins and a piece of the invader DNA (termed the protospacer, shown in red) is selected to be integrated as a new spacer into the CRISPR locus. A prerequisite to be selected as a new spacer is the presence of the PAM sequence (shown in light blue) adjacent to the protospacer. In Haloferax the PAM sequence has to be located upstream of the protospacer sequence (directly 5′ to it).