NEW YORK (MarketWatch)– Shares of Wall Street investment banking giant Morgan Stanley /quotes/comstock/13*!ms/quotes/nls/ms (MS 27.00, -1.38, -4.85%) fell about 4% on Wednesday morning after the Wall Street Journal reported that federal prosecutors are conducting a preliminary investigation of the firm’s mortgage trading business. Morgan Stanley told the paper is has not been contacted by the government regarding a probe, and its Chief Executive James Gorman reiterated that point at a press conference on Wednesday in Tokyo, the paper said. Morgan Stanley shares fell 3.98% to $28.38 in preopen trading.

After GS, it’s MS’ turn to go under the microscope.