U.S. stocks rallied on Monday as a new poll suggested the UK may remain in the European Union, ahead of the critical referendum on Thursday. The three major indexes gained over 1 percent. Meanwhile, shares of Anthem (ANTM - Get Report) and Cigna (CIG - Get Report) were in focus after reports said U.S. regulators remained worried about their proposed $48 billion merger, according to The Wall Street Journal, citing concerns about competition. Walt Disney (DIS - Get Report) shares rose after its movie 'Finding Dory' brought in $136.2 million over the weekend in North America box office sales, making it the highest-grossing animated film debut ever. TheStreet's Scott Gamm reports from Wall Street.