What type of enzyme is known for forming double-strand breaks in DNA at specific target sites?

The Cas9 enzyme is a type of endonuclease that plays a central role in the CRISPR-Cas9 gene-editing technology. It is designed to create double-strand breaks in DNA at precise locations dictated by the sequence of a guide RNA. This targeted action allows for specific modifications of the genome, making it a powerful tool for genetic engineering.

The ability of Cas9 to introduce these breaks at targeted sites is what distinguishes it from other enzymes listed. DNA polymerases are primarily involved in synthesizing DNA strands during replication rather than cutting them. Restriction enzymes do cut DNA, but they typically do so at specific palindromic sequences rather than being guided by RNA. Ligase enzymes serve the function of joining DNA fragments together, which is the opposite of what Cas9 does. Thus, Cas9's unique function in generating double-strand breaks at specific target sites is what makes it the correct answer in this context.