The 727 also proved popular with cargo and charter airlines. FedEx Express introduced 727s in 1978. The 727s were the backbone of its fleet until the 2000s; FedEx began replacing them with Boeing 757s in 2007. Many cargo airlines worldwide employ the 727 as a workhorse, since, as it is being phased out of U.S. domestic service because of noise regulations, it becomes available to overseas users in areas where such noise regulations have not yet been instituted. Charter airlines Sun Country, Champion Air, and Ryan International Airlines all started with 727 aircraft.