What triggers some immune systems to run amok, causing conditions like some types of arthritis? Here, researchers have a lead on one possible root cause: "a specific gene called NOD2 triggers arthritis or makes it worse when leftover remnants of bacteria cell walls, called muramyl dipeptide or MDP, are present. ... Despite recent advances in the treatment of arthritis, none target its cause. Our work with MDP and NOD2 is a step toward understanding the root cause of arthritis which one day may allow certain forms of arthritis to be prevented altogether. ...[Researchers] made this discovery through experiments using two groups of mice, one group was normal and the other had been genetically modified so that their NOD2 gene was deactivated (commonly referred to as 'knocked out'). Then they administered MDP to the joints of mice in each group, and unlike the normal group of mice, the mice with the deactivated NOD2 gene did not experience signs of arthritis. ... Now that we know that bacterial products can activate this NOD2 pathway and that this signal contributes to arthritis, the next step is to find treatments that either rid the body of this inflammatory signal or mask it. Either way, the net effect would be the same: people would be spared from a very crippling disease."