Stock futures on Tuesday were adding to Monday's gains as investors awaited comments from Federal Reserve Chair Janet Yellen and hopes of a "remain" vote in the "Brexit" referendum continued to fuel gains. 

S&P 500 futures were up 0.4%, Dow Jones Industrial Average futures climbed 0.31%, and Nasdaq futures added 0.44%. 

Yellen will speak to the Senate Banking Committee in Washington on Tuesday morning, followed by the House Financial Services Committee on Wednesday. The address before lawmakers will cover monetary policy and the overall health of the U.S. economy.

With little in the way of game-changing data between now and then, Yellen's comments are fairly predictable. The Fed chair expressed her confidence in the U.S. economy after the central bank's meeting last Wednesday, noting that first-quarter weakness appeared temporary. Yellen also cautioned investors not to overact to one piece of data, such as the weak May jobs report.

The higher chances of a vote for the U.K. to remain in the European Union 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) was slightly higher in premarket trading Tuesday after boosting 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) was on watch after announcing 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.

Neustar (NSR) rose 2.4% in premarket trading 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 4.3% before the bell 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 more than 3% 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)   rose 0.8% after signing 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.