Description The ureido-2,2´-diacetic acid bis(hydroxysuccinimide) ester, is the shortest version of MS-cleavable urea-based lysine-lysine reactive homobifunctional cross-linkers, featuring a C2-arm. At neutral or slightly basic pH, it irreversibly crosslinks the neighbouring lysine groups. The presence of the symmetrical urea moiety which is prone to collision-induced dissociation, allows for the unambiguous distinguishing of crosslinks in tandem CID-MS experiments.