What happened

Quantum-Si (QSI 2.48%) stock saw some big pricing swings in Friday's trading. While the protein-sequencing technology specialist's share price closed out the day down 6.2%, it had previously been up as much as 12.8% in the daily session, according to data from S&P Global Market Intelligence.

Quantum-Si started Friday with a bang following news that Ark Invest had purchased 1.4 million shares of the company's stock the day prior. Ark Invest is led by CEO Cathie Wood, and news that the high-profile growth investor has purchased a stock for her company's exchange-traded funds (ETFs) can sometimes create additional bullish momentum for that company's share price. 

So what

Ark Invest publicly discloses its stock positions on its websites, and regular updates for the portfolio compositions of its funds show what stocks Wood's company has bought and sold. With an update published Thursday, the investment firm revealed that it purchased roughly $3.7 million worth of Quantum-Si shares for its ARK Genomic Revolution ETF (ARKG 0.74%). It wasn't a new position for the ETF.

Wood's company began buying Quantum-Si stock when the company went public in June 2021, and it looks like the famous investor is becoming increasingly bullish on the little-known biotech player. Thursday's purchase represented Ark's biggest-ever purchase of the stock. 

But while news that Ark was adding to its position in the company was initially enough to drive big gains for Quantum-Si in Friday's trading, the stock wasn't able to hold on to the gains. Some investors may have sold the stock after the pop in order to record some quick gains. After starting the day with positive momentum, the broader market also pulled back some. The S&P 500 index ended the day down 0.1%, while the Nasdaq Composite index fell roughly 0.2%.

Even after today's pullback, Quantum-Si's share price still closed out this week's trading up 17.6%. 

Now what

While Cathie Wood and Ark are warming up to Quantum-Si, the stock still represents a relatively small holding in the ARK Genomic Revolution ETF. Valued at roughly $37 million, the biotech stands as the 23rd-largest overall stock position in the fund.

Quantum-Si has just started delivering its semiconductor-based protein sequencing platform to customers and recording its first sales. In the first quarter, the company recorded revenue of $254,000, orders from customers totaling $449,000, and a net loss of $23.6 million. With the company still in the early stages of manufacturing and marketing its innovative sequencing platform, it's hard to map where the business will head next, but Wood is betting the high-risk, high-reward stock could be a winner.