Stocks fluctuated mid-morning as Federal Reserve Chair Janet Yellen warned of "considerable uncertainty" over the economic outlook. 

The S&P 500 and the Dow Jones Industrial Average were trading at the flatline, and the Nasdaq fell 0.03%.

The Fed will continue with a "cautious approach" to interest rates, particularly with a potential "Brexit" muddying its outlook, Yellen said in testimony to the Senate Banking Committee in Washington. Yellen said there would be "significant economic repercussions" if the United Kingdom chooses to leave the European Union.

Yellen also will speak before the House Financial Services Committee on Wednesday. The address before lawmakers will cover monetary policy and the overall health of the U.S. economy.

The higher chances of a vote for the U.K. to remain in the EU had global markets in a celebrating mood on Monday. However, trading could still be volatile between now and Friday's results with the outcome still too close to call. A weekend poll showed 45% of voters wish to "remain" while 42% prefer to "exit." An exit from the EU would incite economic and political instability.

Billionaire trader George Soros warned that a U.K. vote to leave the EU could lead to a "Black Friday," wherein the pound would suffer more than Britain's abrupt exit from the European exchange rate mechanism in 1992. While currency speculators might gain, a "Brexit" vote would leave "most voters considerably poorer," Soros said in editorial in The Guardian.

Lennar (LEN - Get Report)  boosted both profit and revenue in its recent quarter. The homebuilder benefited from continued strength in housing demand, particularly in an environment of low interest rates and tighter inventory levels. Earnings of 95 cents a share beat estimates of 86 cents, while revenue rose 15% to $2.75 billion.

CVS Health (CVS - Get Report)   announced plans to expand its healthier foods and beverages offerings to more than 2,900 stores. The drugstore chain has focused on distinguishing itself as the healthier alternative to other pharmacies after banning tobacco sales at its stores in 2014.

Helmerich & Payne (HP - Get Report) was upgraded to overweight from sector weight with an $80 price target at Keybanc. The firm said the company is best positioned for an upturn in domestic land rig activity. The stock rose 0.6%.

Neustar (NSR)  gained 5% after announcing plans to split into two publicly traded companies. The tax-free spinoff will separate the company's information services from its order-management services, the latter of which will retain Neustar's name and brand.

Planet Fitness (PLNT - Get Report) fell 6% after announcing a secondary offering. The fitness chain said a number of existing stockholders intend to offer up for sale 10 million shares of Class A common stock. The offering also grants underwriters a 30-day option to purchase up to an additional 1.5 million shares.

Carmax (KMX - Get Report) fell 4.1% after missing analysts' estimates on its top- and bottom-line. The auto parts retailer earned an adjusted 86 cents a share in its recent quarter, 6 cents below estimates. Same-store unit sales rose 0.2%, below estimates of 0.3% growth.

Boeing (BA - Get Report)  has signed an agreement with Iran Air "expressing the airline's intent" to buy its aircraft. The agreement is the Iran's first major deal with an American company following the nuclear accord, which lifted sanctions, which was reached last year.