The week of Thanksgiving in the U.S. is supposed to be a slow one but nobody told the internet's favorite investor. Cathie Wood scooped up dozens of her favorite stocks for Ark Invest's exchange-traded funds (ETFs) last week.

This has been a tough year for tech stocks across the board, and especially hard on shares of Exact Sciences (NASDAQ:EXAS), Adaptive Biotechnologies (NASDAQ:ADPT), and Surface Oncology (NASDAQ:SURF). Here's why the losses haven't stopped Wood from buying each of them multiple times last week.

Investors picking new stocks to buy.

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Exact Sciences

Over the years, Ark Invest's exchange-traded funds have accumulated so many shares of Exact Sciences the firm now owns more than 7% of the company. Every day the market was open last week, Ark Invest bought more shares of the clinical diagnostics company.

Exact Sciences was a big performer for Ark Invest ETFs in 2020 thanks to soaring COVID-19 testing revenue. Rapidly declining COVID-19 revenue has been coupled with a stock price drop of around 58% since the stock peaked in January.

Fortunately, COVID-19 testing is a relatively small part of Exact Sciences' overall business. Revenue from the company's non-invasive colon cancer test, Cologuard, increased 31% year over year in the third quarter. Precision oncology revenue derived from the company's Oncotype DX products soared 59% year over year to $145 million.

Exact Sciences' screening and precision oncology sales are rising fast but so are the company's operating expenses. Exact Sciences lost a frightening $167 million in the third quarter. Looking ahead, the company needs to show signs its business can scale or the stock will fall even further.

Doctor and patient.

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Adaptive Biotechnologies

Wood has bought more than a million shares of this diagnostics start-up since the beginning of September. Last week, she upped Ark Invest's stake to more than 4.2% of the company.

Adaptive Technologies is another diagnostics business that can't make ends meet despite rapidly rising revenue from clonoSEQ. This is a hyper-sensitive test to monitor for the recurrence of common blood-based cancer.

Diagnostics is a hyper-competitive industry when test procedures can be easily replicated. Adaptive Biotechnologies' approach could set clonoSEQ far apart from the competition. The company is pioneering the practice of watching the adaptive immune system for tell-tale signs of a blossoming malignancy.

Adaptive Biotechnologies reported clonoSEQ test volume that soared 47% year over year in the third quarter. Despite the soaring popularity of its product, the company lost $56 million in the third quarter.

Doctor with a patient.

Image source: Getty Images.

Surface Oncology

Surface Oncology isn't a large component of Ark Invest ETFs at the moment, but it could be. After making significant purchases last week, the firm now owns around 11% of the pre-commercial stage drugmaker. 

Shares of Surface Oncology soared last December after GlaxoSmithKline licensed SRF813. This is a pre-clinical stage cancer therapy that won't enter human-stage testing until early 2022. Unfortunately, Surface Oncology is down by about 59% since it peaked in January.

This summer, everyone was disappointed to learn that just one cancer patient out of 18 had measurably smaller tumors after treatment with SRF388, the company's lead candidate. The situation doesn't look good for the company's most advanced program, but the stock market could be undervaluing the company's pipeline. 

Surface Oncology's market cap is a sprightly $263 million at recent prices. This December, the company will present data from SRF617, a clinical-stage therapy being tested among patients with advanced solid tumors in a phase 1 trial. Moderately compelling results from this study or any of Surface Oncology's candidates could quickly push its market cap up past the $1 billion mark again.

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