 In addition, all Marine infantry units are prepared, and regularly train, to perform heliborne, or "vertical envelopment" (i.e., air assault) operations when supported by MV-22 medium tiltrotor and/or CH-53 heavy helicopters and mechanized operations (when supported by attached amphibious assault vehicle units).Additionally, some Marine infantrymen (usually only those assigned to reconnaissance or special operations units) attend U.S. Army Airborne or Ranger training.However, since the USMC does not maintain either airborne or Ranger infantry units, only a relatively small number of Marines ever attend these two schools.