CryptoKitties is a game built on Ethereum that allows users collect and breed cryptographically unique creatures called CryptoKitties. Each kitty exists on the Ethereum blockchain as a ERC721 non-fungible token which stores the genome of the kitty. Each kitty has a unique genome that defines its appearance and traits. Players can breed their kitties to create new kitties and unlock rare attributes or "cattributes" as they are referred to in the game.