Yelp Inc. signage is displayed on a monitor on the floor of the New York Stock Exchange.

Check out the companies making headlines in midday trading.

Yelp — Shares of the online reviewing company jumped 5.7% after activist investor TCS Capital Management took a stake in the firm, saying in an open letter the company should explore strategic alternatives, including a sale. TCS built up a more than 4% position in Yelp, becoming one of the company's top five shareholders. TCS said Yelp is "shockingly undervalued" and could be sold to a private equity buyer for at least $70 per share, a more than 120% premium.