* Futures up: Dow 68 pts, S&P 10 pts, Nasdaq 22.25 pts
By Yashaswini Swamynathan
June 21 (Reuters) - U.S. stock index futures were higher on Tuesday ahead of Federal Reserve Chair Janet Yellen's comments on the economy and Britain's vote on its European Union membership on Thursday.
* Yellen is expected to speak on the central bank's monetary policy before the Senate Banking Committee at 10:00 a.m. ET (1400 GMT) in Washington. She will also attend a meeting at 4:30 p.m. ET with other Fed officials.
* The central bank left interest rates unchanged last week and sounded caution after surprisingly weak monthly hiring data put the economy's recovery to question.
* Wall Street, which rallied at the open on Monday, closed higher as investors breathed a sigh of relief as opinion polls showed fewer chances of Britain leaving the European Union.
* Britain is scheduled to vote on Thursday, an event that has roiled the markets in the past week as investors fret over its consequences, including the possibility of the EU falling apart if the country votes to leave.
* Investors have erred on the side of caution in the past week, sticking to safe-haven assets such as gold and the yen.
* Gold fell for the second day and the dollar remained weak as risk appetite increased.
* Opko Health rose 8.3 percent to $9.95 premarket after the U.S. Food and Drug Administration approved a new application for its drug, Rayaldee.
* Homebuilder Lennar rose 0.7 percent to $47 after reporting better-than-expected quarterly profit.
Futures snapshot at 7:20 a.m. ET:
* Dow e-minis were up 68 points, or 0.38 percent, with 20,613 contracts changing hands.
* S&P 500 e-minis were up 10 points, or 0.48 percent, with 156,759 contracts traded.
* Nasdaq 100 e-minis were up 22.25 points, or 0.51 percent, on volume of 19,309 contracts. (Reporting by Yashaswini Swamynathan in Bengaluru; Editing by Don Sebastian)