The M1 version went into service in 1971 and was phased out in favour of the M2 in 1974, itself replaced by the M20 in 1977. The new M4 missile entered service in 1985 and has replaced the M-20. French nuclear-powered submarines (SNLE: "Sous-marin Nucléaire Lanceur d'Engins", "Nuclear Device-Launching Submarine"), are able to carry 16 missiles each. The oldest ship, "Redoutable", was not converted to carry the M-4 and was withdrawn from service in 1991. Logistical support for the MSBS fleet is provided by the Île Longue Naval Base in Brest Bay, where the assembly and storage facilities for maintenance of readiness are located. Three SSBNs are intended to be operational at any one time.