CryptoKitties, the viral Ethereum video game that lets players collect and breed digital cats, are now trading on the OPSkins virtual gaming assets marketplace. The listing enables users to instantly buy and trade the digital cats using US dollars, euros and dozens of other currencies, as well as WAX tokens, a utility token developed by the founders of OPSkins.

“Everyone is talking about CryptoKitties, and not just because they are cute,” said William Quigley, CEO of WAX and CEO of OPSkins. “The concept of a virtual collectible item on the blockchain adds a fresh value proposition to the crypto world and we see OPSkins and WAX as a key player in developing what we call the ‘Crypto Collectibles’ space.”

CryptoKitties is a video game built on the Ethereum network. It is similar to Neopets and allows players to adopt, raise and trade digital pets.

The game has skyrocketed in popularity this past month with rare kitties going for well over US$100,000. At some point, CryptoKitties became the largest decentralized application on the Ethereum protocol, accounting for more than 13% of the network’s transactions.

The craze peaked to the point that initial coin offerings (ICOs) could not launch on the Ethereum protocol due to network congestion caused by the sale and trading of digital kittens on CryptoKitties.

“The popularity of virtual cats fits the euphoria we see elsewhere in the cryptocurrency space,” Peter Atwater, who studies market sentiment and heads Financial Insyghts, told CNBC. “It feels very reminiscent of the Candy Crush craze that helped propel the King Entertainment IPO back at the peak of the ‘Unicorn’ era in mid 2014.”

According to Arthur Camara, an Axiom Zen software engineer on the CryptoKitties team in Vancouver, the purpose of the game was to raise awareness about blockchain technology. The team had a six-month plan to make CryptoKitties a hit. Nobody expected it would take just a week.

“We had big goals but we never expected them to come this fast,” Camara told Motherboard. “We’re definitely surprised that CryptoKitties exploded this way, and we didn’t expect to see people re-selling their kitties for thousands of dollars.”

“It was never our [intention to have CryptoKitties selling for US$100,000]. We want CryptoKitties to be selling for a dollar. It’s crazy when we see these numbers, but we wanted to make it approachable and we wanted to create groups that can define what’s important to them and buy the cat with traits desirable to them for an affordable price.”

CryptoKitties players buy and sell unique digital kittens using ether. With two kittens, players can breed their own digital kittens and sell them on the marketplace. The starting price is set by the player and the price goes down until the end of the auction or the kitten is sold.

Although CryptoKitties is not a cryptocurrency, it does offer something similar to one: each CryptoKitty is one-of-a-kind, 100% owned by the user, validated through the blockchain, and the value can appreciate or depreciate based on the market.

The CryptoKitties team intends to release a new CryptoKitty every 15 minutes until November 2018, whose starting price is set by the average of the last five CryptoKittens that were sold plus 50%.

CryptoKitties utilize Ethereum’s new ERC-721 protocol, a non-fungible token standard.

It represents one of the earliest attempts to deploy blockchain technology for recreational and leisurely purposes, and might have signaled a promising use case for Ethereum: digital collectibles. Following the successful launch of the game last month, similar dapps have emerged such as CryptoPuppies and CryptoPets.