CRISPR-Cas9 gene editing complex, molecular structure. The CRISPR-Cas9 protein (centre) is used in genome engineering to cut DNA (deoxyribonucleic acid). It uses a guide RNA (ribonucleic acid) sequence to cut DNA (helical strand) at a complementary cleavage site. At centre, the blue area represents 'molecular scissors' cutting the DNA strand. In genetics, CRISPR-Cas mechanisms are used in gene editing. The cell nucleus (round) is seen in the background.