Toxic component of a type II toxin-antitoxin (TA) system. Expression in E.coli inhibits cell growth; bacteriostasis is neutralized by expression of cognate antitoxin Xre. Expression in E.coli leads to almost complete depletion of intracellular NAD(+): NAD(+) levels are partially restored when coexpressed with antitoxin Xre. Homodimer. Forms a complex with cognate antitoxin Xre. Belongs to the MbcT/ParT/Res family.