The story says blood-testing firm Theranos – headed by 31-year-old Elizabeth Holmes, who is reputed to be the world’s youngest self-made billionaire – doesn’t use its own technology for many of its tests. The story also raises questions about the accuracy of the testing technology.

Holmes has achieved rock-star status since Fortune put her on the cover in June of last year. Her company is valued at $9 billion – she owns roughly half – and her board includes the likes of former Secretaries of State Henry Kissinger and George Schultz. She has touted her technology as being able to process “a full range of blood tests from a few drops of blood.”

The Journal story was written by John Carreyrou, who is an experienced investigative reporter specializing in medical issues. I have no independent information on the story, but can vouch for the quality of Carreyrou’s work.

Holmes declined comment to the Journal, but overnight tweeted a link to her web site defending her testing technology. Her high-powered lawyer, David Boies, told the Journal the transition to doing all tests with the company’s new technology was “a journey.”

As the economy has slowed, speculation about the fate of unicorns – the roughly 130 venture-backed start ups with valuations exceeding $1 billion – has grown. Fortune’s Erin Griffith reported recently that many venture capitalists keep “dying unicorn” lists, although they won’t reveal which companies are on them.

There’s a good chance Theranos now joins that herd.

Enjoy the day. More below.

Top News

• Walmart warns of weak profits

Shares of the world's largest retailer suffered their greatest drop in 15 years after Walmart issued a warning it no longer expected sales growth for the current fiscal year. It also told investors that big spending on e-commerce efforts and giving workers raises would weigh on profits in the next two years. One way Walmart hopes to turn around the business is stock more goods that will appeal to middle and upper middle class shoppers, a pivot away from the company's massive reliance on lower-income consumers today. Fortune

• Square files for an IPO

Square, the electronic payment company led by founder and Twitter CEO Jack Dorsey, has filed for an initial public offering with plans to raise $275 million. For the first six months of the year, the filing showed Square reported a $77.6 million net loss on around $560.5 million in net revenue. The company also acknowledged Dorsey could become distracted by holding two demanding jobs. Fortune

• FBI probes fantasy sports sites

The FBI and the U.S. Department of Justice have reportedly launched inquiries into daily fantasy sports sites DraftKings and FanDuel, just over one week after a scandal surfaced involving insider play on those websites. Since a scandal erupted involving allegations an employee used insider data to win big money, the press has looked deeply into the industry – and the major sites all have quickly moved to ban their employees from playing on any daily fantasy sites while this all gets sorted out. Fortune

• Netflix's stock hits speed bump

Netflix shares tumbled by as much as 14% in after-hours trading on Wednesday after the popular video-streaming website provider reported lower-than-expected quarterly growth numbers. Revenue grew more than 23% to $1.74 billion, but that metric, along with profit, missed Wall Street's targets. There was also an issue with subscriber count, which Netflix attributed to some difficulties processing some payments due to the shift by credit-card companies to chip-based cards. Fortune

• Burberry stung by China

Another high-profile stock taking a dive is luxury-goods maker Burberry, which this morning saw shares fall by the most since 2012 after it warned profit would probably decline for the second straight year. The company has been hurt by slowing demand in China and Hong Kong. Burberry generates more than 30% of revenue from Chinese shoppers but only 2% from Japan – which is where big-spenders from China have been heading to take advantage of the exchange rate. Bloomberg

Around the Water Cooler

• Nike's big sales promise

The world's largest athletic apparel and shoes maker says it expects to generate $50 billion by 2020, adding some $20 billion in sales in an ambitious five-year period that analysts have said looks achievable. How can it get there? E-commerce is a big opportunity, Nike says, as is more sales to women and the expansion of the Jordan brand. The growth targets Nike and rival Under Armour have outlined recently imply that athletic apparel will remain trendy and continue to perform well in the global apparel market. Fortune

• Toyota's hydrogen-based society

The Japanese automaker on Wednesday outlined some ambitious environmental targets, promising to cut emissions generated by production to roughly half of 2001 levels by 2020. Emissions generated by new vehicles will be cut by 22% by the end of the decade. How does Toyota hope to achieve these goals? A bet on hydrogen and hybrid cars when other automakers are investing more into electric vehicles. Fortune