PAM Sequence

The PAM sequence (Protospacer Adjacent Motif) is a short, conserved DNA sequence adjacent to the target site that is required for Cas nuclease binding and cleavage. Without a matching PAM, even a perfectly complementary guide RNA cannot direct Cas9 to its target. The PAM is not part of the guide RNA's recognition sequence; it is a separate constraint that limits where CRISPR edits can occur.

The canonical PAM for Streptococcus pyogenes Cas9 is NGG, where N is any nucleotide. Different Cas orthologs recognize different PAMs — Cas12 prefers TTTV, Cas13 targets RNA without a PAM — expanding the editable genome but introducing new design constraints. The PAM is both a limitation and a safety feature: it prevents the bacterial CRISPR system from cutting its own spacer array, and it partially constrains off-target activity in engineered systems.

From an information-design perspective, the PAM is a address-space constraint: it defines which genomic locations are reachable by a given CRISPR system. Engineers cannot edit arbitrary sequences; they can only edit sequences adjacent to recognized PAMs. This constraint is not incidental. It shapes the entire design space of CRISPR applications and has driven the search for Cas variants with relaxed or alternative PAM requirements.

The PAM is a reminder that even programmable systems have fixed interfaces — and that the boundary between what is editable and what is untouchable is itself a design choice encoded in protein evolution, not human intention.