CRISPR-associated endonuclease complexed with a complementary DNA substrate, molecular model. The image shows the CRISPR-associated endonuclease Cas9/Csn1 (grey, the front half is removed to show what is going on inside), the single guide RNA (sgRNA, red), the target strand of 18-nucleotide complementary DNA substrate, PAM-proximal end (blue), the target strand of 18-nucleotide complementary DNA substrate, PAM-distal end (cyan) and the non-target strand of 18-nucleotide complementary DNA substrate (green).