Targeting a DNA sequence for editing by the CRISPR‑Cas9 system requires that a protospacer‑adjacent motif (PAM) sequence appear just downstream of the target sequence. For S. pyogenes Cas9 this PAM is the trinucleotide NGG, where N represents any nucleotide. Assuming a fragment of DNA includes equal amounts of A, C, G, and T bases, how many bases apart, on average, would one expect to find the NGG PAM?