"Borderlands" includes character-building elements found in role-playing games, leading the developer Gearbox Software to call the game a "role-playing shooter". At the start of the game, players select one of four characters, each with a unique special skill and with proficiencies with certain weapons. From then on, players take on missions assigned through non-player characters or from bounty boards, each typically rewarding the player with experience points, money, and sometimes a reward item. Players earn experience by killing both human and non-human foes and completing in-game challenges (such as getting a certain number of kills using a specific type of weapon). As they gain levels from experience growth, players can then allocate skill points into a skill tree that features three distinct specializations of the base character; for example, Mordecai can become specialized in sniping, gunslinging with revolvers, or using his pet Bloodwing to assist in kills and health boosting. Players can distribute points among any of the specializations, and can also spend a small amount of in-game money to redistribute their skill points.