In cell nucleus, DNA is wound around cores made of histone proteins. One of the histone proteins, H2AX, plays important role in recognition of DNA damage. This schematic shows what the researchers suspect is happening to DNA molecules when exposed to strong pulses of THz radiation. The upper row is immediately following the pulse when the strand of DNA is broken. The lower portion shows the H2AX molecules around the break site 1 - 3 minutes later after they are "flagged" by the attachment of a phosphate group, which signals that DNA damage has taken place.