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CryptoKitties is a game built on the Ethereum blockchain in which users can collect and trade digitally scarce virtual cats.


CryptoKitties is a blockchain based game in which users collect and trade digitally scarce virtual cats using ETH tokens. Each kitty has a unique 256-bit genome which defines it's appearance and traits. Players are able to breed new kitties who inherit traits from their parents' genome. AxiomZen developed the game including the five smart contracts which allow users to interact with the Ethereum blockchain. The blockchain tracking of the CryptoKitties means that the kitties are permanently and immutably available on the blockchain regardless of the original creators continued support of the project. CryptoKitties is a new type of digital asset known as a cryptocollectables, other examples include CryptoPets, CryptoPuppies.

Supply and breeding

The supply of kitties is limited based a fixed number of new kitties called Gen 0 kitties released every fifteen minutes. Additionally, users can choose a mother and father to breed a genetically unique offspring. The rate at which users can breed new kitties is limited, and users also have to pay a fee to breed.


Each kitty has a unique genome represented by a 1 or a 0 at each position of their 256-bit genome. The exact genome of the kitty is stored on the blockchain represented by this string of bits. This genome determines the properties of the kitty such as background color, breeding cooldown time, whiskers, beard, and others. Players can sire a new offspring from a chosen set of parents. This is done by a genetic algorithm which mixes the genetic data from the parents as well as info from the ethereum block hash to create a unique offspring. The offspring may have unique or desirable properties to players which make them more desirable for trading on the Cryptokitties marketplace.

A cracking program was developed which allows users to determine the exact genes of the offspring for a given 256 block time segment. If the user doesn't like that specific gene set they could wait 256 blocks and get another iteration based on the block hash and so on until they arrived at a desired gene set.


Buying and selling of cryptokitties occurs on a marketplace using the digital wallet called MetaMask.


By December 3, 2017 more than $1 million dollars worth had been transacted through the game and was the most popular decentralized application on the Ethereum network, responsible for 15% of all traffic. The network congestion caused by the game highlighted the blockchain scalability issues for the Ethereum network and blockchains in general.

CryptoKitties is a blockchain based virtual game developed by Axiom Zen that allows players to purchase, collect, breed and sell various types of virtual cats. It represents one of the earliest attempts to deploy blockchain technology for recreational and leisurely purposes. The game's popularity in December 2017 congested the Ethereum network, causing it to reach an all-time high in transactions and slow down significantly.On March 20, 2018, it was announced that CryptoKitties would be spun off into its own company and raised $12 million from several top venture capital firms and angel investors. The investment round was led by New York based Union Square Ventures and San Francisco based Andreessen Horowitz.


CryptoKitties is not a cryptocurrency. Instead it operates on Ethereum's underlying blockchain network, as a non fungible token (NFT) unique to each CryptoKitty. Each CryptoKitty is unique and owned by the user, validated through the blockchain, and its value can appreciate or depreciate based on the market. CryptoKitties cannot be replicated and cannot be transferred without the user's permission even by the game developers. Users can interact with their CryptoKitties, having the ability to buy, sell, and sire (breed) them. A test version of CryptoKitties was unveiled at ETH Waterloo on October 19, 2017, the largest Ethereum hackathon in the world. As of December 2, 2017, Genesis, the first and highest selling cat was sold for 246.9255 ETH (~$117,712 USD) on that day.

The virtual cats are breedable and carry a unique number and 256 bit distinct genome with DNA and different attributes (cattributes) that can be passed to offspring. Several traits can be passed down from the parents to the offspring. These include pattern, mouth (shape), fur, eye shape, base color, accent color, highlight color, eye color, and optional wild and environment characteristics. Other features like cool down times are not passed down but are instead a function of the 'generation' of the offspring, which is one higher than the maximum generation between the two parents.


A CryptoKitties' ownership is tracked via a smart contract on the Ethereum blockchain. CryptoKitties are distributed automatically, via smart contract, at the rate of one every 15 minutes (672 per week) for one year. Each CryptoKitty is represented in the form of a non-fungible ERC-721 token, which allows for each entity to have specific cattributes.

Based on the limited number of cats going into circulation and their limited genomes, there is a limit of 4 billion total cats that can be bred. Each cat will have a distinct visual appearance ("phenotype") determined by its immutable genes ("genotype") stored in the smart contract. Because cats are tokens on a blockchain, they can be bought, sold, or transferred digitally, with strong guarantees of ownership. A CryptoKitty does not have a permanently assigned gender. While they can only engage in one breeding session at one time, each cat is able to act as either matron or sire. The owner chooses with each breeding interaction.

A group known as Axiom Zen innovation studio developed the game. Until November 2018, Axiom Zen intends to continually release a new CryptoKitty every 15 minutes, with the rest of supply determined by breeding of CryptoKitties. CryptoKitty owners may put them up for sale via an auction for a price set in Ether (ETH).


There are concerns that Cryptokitties is crowding out more serious, significant business that use the Ethereum platform. As of December 5, 2017 Etherscan has reported a sixfold increase in pending transactions on Ethereum since the game's release just a week earlier. "CryptoKitties has become so popular that it's taking up a significant amount of available space for transactions on the Ethereum platform", said Garrick Hileman, from the University of Cambridge.

Ethereum miners increased the gas limit in response to CryptoKitties, which allowed for more data per block and increasing transactions per second.

In December 2017 a CryptoKitty sold for $100,000.In May 2018, CryptoKitties launched their first celebrity-branded CryptoKitty with Stephen Curry, an American professional basketball player. As part of the partnership, Curry was given three CryptoKitties with special imagery, the first of which he put up for auction. The company later suspended the auction, claiming that Stephen Curry wasn't as involved as they initially thought. The company was later sued for trade secret theft over the Stephen Curry collectibles.In 2018 CryptoKitties will help the german museum ZKM Center for Art and Media Karlsruhe explain blockchain.


September 2019
CryptoKitties raises a $11,000,000 series B round from Digital Currency Group, Warner Music Group and a16z crypto.
March 20, 2018
Union Square Ventures invests in CryptoKitties

Fred Wilson announces on the USV blog that Union Square Ventures invested in a CryptoKitties funding round.

March 2018
CryptoKitties raises a $12,000,000 series A round from Andreessen Horowitz and Union Square Ventures.

Funding rounds



Dieter Shirley


Fabiano PS


Mack Flavelle


Roham Gharegozlou

Further reading


Do You Really Own Your CryptoKitties?

Greg McMullen


How a Hackathon Birthed the CryptoKitties Origin Story

Rachel Wolfson


Introducing 'CryptoKitties,' the New Digital Pets Taking Ethereum by Storm

Joseph Hincks


People have spent over $1M buying virtual cats on the Ethereum blockchain

Fitz Tepper


The ethereum world is now obsessed with breeding cartoon cats

Joon Ian Wang


Towards Cracking Crypto Kitties’ Genetic Code

Mo Dong


Documentaries, videos and podcasts


How to Chase Fancy Cats

May 22, 2018

How to Play CryptoKitties

April 25, 2018

The Future of CryptoKitties

October 31, 2018




Gian Volpicelli, Wired UK
February 27, 2021
Non-fungible tokens provide a way to invest in and own digital imagery. But is it just another crypto fad--or the future of intangible art?


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