Some regions of our genomes are under permanent lockdown because they are hazardous to our health - or at least the health of our future offspring. These secured regions include large swaths of parasite-infested DNA - DNA that contains transposable elements, virus-like genetic parasites that have the ability to hop around the genome and cause harmful mutations.
Because out of control transposable elements are a major danger, cells (ours and those of most other organisms) have an elaborate maximum-security system for shutting these bad boys down. Just how this lockdown system works is an active area of research, and a recent
paper revealed how plant cells enforce security and prevent prison breaks by these DNA parasites.