"but they're nice and juicy, and Riot's one of them right now,"

Short-seller Andrew Left can't find enough ways to short, or bet against, bitcoin. "If you're a short-seller right now and you're cautious and you want to short something that's related to bitcoin and not bitcoin, you have limited opportunities," the Citron Research founder said Tuesday on CNBC's "Fast Money." "But they're nice and juicy, and Riot's one of them right now," he said. Citron Research tweeted Monday afternoon that it was shorting Riot Blockchain, a biotech company that added "blockchain" to its name in October and saw its shares surge. The stock is now up more than 900 percent this year and has a market cap of about $372 million. Riot turned down a CNBC request to appear on "Fast Money." "No one at the company has been able to get in touch with the CEO," a Riot spokesperson said. Based on his "sum of parts" analysis, Left said on CNBC the stock is worth $9 a share, down nearly 77 percent from where it closed on Tuesday. Shares fell 6 percent in after-hours trading after rising 5.8 percent to $38.60 a share Tuesday.

Andrew Left, Citron Research Michael Newberg | CNBC

Riot Blockchain announced Tuesday it raised $37 million in gross proceeds from subscription agreements with accredited investors for Riot shares, with Canaccord Genuity as financial advisor. The proceeds will be used for bitcoin mining and other investments, Riot said. Left said he is using options to bet against Riot. He said the company is "even worse" than the initial coin offerings that have raised millions of dollars each based primarily on plans laid out in online whitepapers. Riot has just enough blockchain-related investments to announce them publicly, "the whole time really misrepresenting the fact that nothing they have is a real player in the crypto industry," Left said. The company used to focus on blood testing and animal health care. While Riot says on its website that it maintains its Bioptix business, the company intends to "gain exposure to the blockchain ecosystem through targeted investments in the sector, with a primary focus on the Bitcoin and Ethereum blockchains."

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