To maintain the integrity of the genome, nuclear division (mitosis) must not occur if the DNA is damaged. To prevent this, Cdc25, an essential regulator of mitosis, is inactivated, and we now know how. DNA damage activates Chk1, which adds a phosphate group to Cdc25. This creates a binding site for Rad24, which then transports Cdc25 out of the nucleus where it cannot drive the cells into mitosis.