The nitrogen mustards are cytotoxic chemotherapy agents similar to mustard gas. Although their common use is medicinal, in principle these compounds can also be deployed as chemical warfare agents. Nitrogen mustards are nonspecific DNA alkylating agents. Nitrogen mustard gas was stockpiled by several nations during the Second World War, but it was never used in combat. As with all types of mustard gas, nitrogen mustards are powerful and persistent blister agents and the main examples (HN1, HN2, HN3, see below) are therefore classified as Schedule 1 substances within the Chemical Weapons Convention. Production and use is therefore strongly restricted.