This is an illustration of what happens when viral DNA enters the nucleus of a cell with low dUTP levels (left) versus high dUTP levels (right). A team of researchers based at Johns Hopkins has decoded a system that makes certain types of immune cells impervious to HIV infection. The system's two vital components are high levels of a molecule that becomes embedded in viral DNA like a code written in invisible ink, and an enzyme that, when it reads the code, switches from repairing the DNA to chopping it up into unusable pieces. The researchers, who report the find...