Another type of DNA damage is “Double-strand DNA break” where both backbones of the DNA double helix are snapped - either by external factors like radiation or internal ones like reactive oxygen molecules.

For this type of damage, homologous recombination repair or non-homologous end joining (NHEJ) will take place.

Homologous recombination repair includes the invasion of an undamaged DNA molecule by a damaged molecule of identical or very similar sequence.

While NHEJ is a repair mechanism that directly ligates broken ends of DNA DSBs using a heterodimeric enzyme complex consisting of the proteins Ku-70 and Ku-80, the catalytic subunit of DNA protein kinase (DNA-PKCS), and XRCC4/ligase IV.