Stocks making the biggest moves premarket: TWTR, REGN, YUM, GRUB, CVS, TMUS & more

Check out which companies are making headlines before the bell:

Twitter – Twitter earned an adjusted 19 cents per share for the fourth quarter, 5 cents a share above estimates. Revenue came in well above Street forecasts. Monthly active users were up four percent over a year ago, though flat compared to the prior quarter. Daily active users were up 12 percent from a year before.

Regeneron – Higher sales of the company's flagship eye drug Eylea helped Regeneron beat estimates on both the top and bottom lines. Regeneron earned an adjusted $5.23 per share, compared to estimates of $4.53 a share. Revenue beat expectations, as well.

Yum Brands – The restaurant operator posted adjusted quarterly profit that was 16 cents a share above estimates at 96 cents per share. Revenue missed forecasts, however, and comparable-store sales at Pizza Hut and Taco Bell were below estimates. KFC sales were slightly better than expected. Separately, Yum announced it would buy a $200 million stake in food-delivery service GrubHub.

CVS Health – CVS beat forecasts by 3 cents a share, with adjusted quarterly profit of $1.92 per share. Revenue topped estimates, as well. Comparable-store sales dropped by seven-tenths of a percent, however. CVS also announced pay raises for workers, crediting the positive financial impact of tax reform.

Viacom – Viacom posted a mixed quarter, beating bottom-line estimates by 9 cents a share at an adjusted $1.03 per share. Revenue fell short of forecasts, hurt by a drop in fees that the company collects from cable operators.

T-Mobile US – T-Mobile reported adjusted quarterly profit of 48 cents per share, 9 cents a share above estimates. Revenue missed Street projections, however. The wireless service provider also said thanks to the new tax law, it doesn't expect to be a material cash tax payer until 2024, four years later than earlier forecasts.

21st Century Fox – Fox beat estimates by 4 cents a share, with adjusted quarterly earnings of 42 cents per share. Revenue topped forecasts, as well, with results helped by increases in cable and satellite revenue.

Tesla – Tesla reported an adjusted quarterly loss of $3.04 per share, smaller than the loss of $3.12 forecast by analysts. Its overall loss was its biggest ever. Revenue was roughly in line with forecasts, and the automaker said it was sticking to recently revised production forecasts for its model three. Tesla also said that its spending could rise this year.

iRobot – The shares are under pressure after both earnings and its outlook fell below Street estimates. The maker of robotic vacuum cleaners did say it expects to surpass the $1 billion revenue mark this year for the first time.

Yelp – Yelp said it would operate at a loss for 2018, due to expenses related to marketing and increased investments in its restaurant reservations system.

Athenahealth – Athenahealth named former General Electric CEO Jeff Immelt as its chairman. He replaces Jonathan Bush, who will continue to serve as CEO. Immelt assumes the role in the midst of a shakeup at the health-related software company, which began after activist investor Elliott Management took a stake in the company last year.

Take-Two Interactive Software – Take Two forecast current-quarter revenue below Street forecasts. It did report a profit for the holiday quarter thanks to a gain related to the new U.S. tax law.

Yum China – Yum China reported adjusted quarterly profit of 19 cents per share, beating forecasts by 1 cent a share. Shares of the operator of KFC, Pizza Hut, and Taco Bell restaurants in China were pressured after Pizza Hut comparable-store sales came in below analysts' forecasts.

Allstate – Allstate reported adjusted quarterly profit of $2.09 per share, easily beating the consensus estimate of $1.53 a share. The insurer's revenue also beat forecasts. Allstate noted strong home and auto insurance profit margins, among other factors.

Walmart – The retailer has asked vendors for more merchandise priced at $10 and up to sell online, according to a Reuters report, in an effort to make its online business profitable.

BlackRock – The asset management firm will reportedly raise about $10 billion for a new business that would make direct equity investments in companies, according to Reuters.

Gilead Sciences – Gilead won Food and Drug Administration approval for its HIV combination drug known as Biktarvy. That prompted an immediate patent infringement lawsuit from rival drug maker ViiV Healthcare, a joint venture of GlaxoSmithKline, Pfizer, and Shionogi Ltd.

Amazon.com – Amazon will begin delivering groceries from its Whole Foods operations via its Prime Now service. The one-hour and two-hour service will launch in Austin, Cincinnati, Dallas, and Virginia Beach, and expand across the country later this year.