The highest level in sports car racing, these cars are purpose-built racing cars with enclosed wheels, and either open or closed cockpits. Since the World Sportscar Championship was conceived there have been various regulations regarding bodywork, engine style and size, tyres and aerodynamics to which these cars must be built. Sports-prototypes may be (and often are) one-of-a-kind machines, and need bear no relation to any road-going vehicle, although during the 1990s some manufacturers exploited a loophole in the FIA and ACO rules which meant cars racing in the GT category were actually true sports-prototypes and sired some road-going versions for homologation purposes. The Dauer-Porsche 962LM, Porsche 911 GT1-98, Mercedes CLK-GTR and Toyota GT-One were prime examples of prototypes masquerading as GTs.