White House spokesman Josh Earnest says President Obama was briefed on the technical problem that has halted trading on the New York Stock exchange by White House counterterrorism and homeland security adviser Lisa Monaco and chief of staff Denis McDonough. Earnest said the NYSE has been in close contact with the Department of Homeland Security, the Securities and Exchange Commission and the Treasury Department. The halt in trading on the New York Stock Exchange hasn't stopped trading of NYSE-listed shares, which continue to change hands on other exchanges. Tom Caldwell, who runs an investment firm with stakes in several exchanges, says there are some 60 exchanges and trading venues that can take orders when one goes down, and so he isn't worried. The technical problem that has halted trading on the New York Stock Exchange hasn't changed the Dow or other market indexes much. Traders worried about China's failure to stop a steep drop in its own market, and they were also monitoring a logjam in talks between Greece and its creditors. In May 2010, the Dow Jones industrial average plunged hundreds of points in minutes due to mass selling triggered by computerized trading programs. In March 2012, BATS Global Markets, a Kansas company that offers stock trading services, canceled its own IPO after several technical snafus. Two months later, a highly anticipated IPO of Facebook on the Nasdaq exchange was marred by a series of technical problems, rattling investors unsure if their orders to trade went through.