From the GBC game case:
Cartridge Only Professional boxing comes to the Game Boy Color with KNOCKOUT KINGS. Choose from 38 professional boxers from all weight classes including Evander Holyfield, Ike Quartey, Lennox Lewis, Michael Moore, Shane Mosley, Oscar De La Hoya, "Sugar" Ray Leonard, and Muhammad Ali. For some quick action, choose any two boxers and enter an Arcade match. Arcade matches are less realistic and can result in quick knockouts. However, you can also choose to play in a simulation Exhibition match, where you must master ring strategies and wear down your opponent. If you are looking for a real challenge, create a boxer and fight through the ranks and become world champion. KNOCKOUT KINGS brings the subtlety of the sweet science to your handheld, without having pay outrageous pay-per-view prices.