US Securities and Exchange Commission chief Mary Schapiro announced Monday that she will step down, nearly four years after taking charge of the key markets regulator in the middle of the financial crisis. Schapiro will depart as SEC chairman on December 14 credited with expanding the SEC\'s reach into derivatives and hedge funds while helping Wall Street recover both in strength and confidence. Her record also includes a key role in expanding and deepening regulation, mainly through the Dodd-Frank legislation that has riled her charges in the financial industry. President Barack Obama quickly designated current SEC commissioner Elisse Walter to fill in for Schapiro. Walter, a Democrat, has been on the SEC board since 2008; before that she worked at the Financial Industry Regulatory Authority (FINRA). It was not clear whether Obama intended to nominate Walter to replace Schapiro as SEC chairperson, which would require the approval of the Senate. \"It has been an incredibly rewarding experience to work with so many dedicated SEC staff who strive every day to protect investors and ensure our markets operate with integrity,\" Schapiro said in a statement. \"Over the past four years we have brought a record number of enforcement actions, engaged in one of the busiest rulemaking periods, and gained greater authority from Congress to better fulfill our mission.\" Schapiro took over the SEC in January 2009, appointed by Obama just after he took office. She came in as financial markets were in freefall and numerous investment banks, securities houses and commercial banks were foundering. She also faced deep hostility from Wall Street and criticism from the financial industry\'s backers in Congress. Under her guidance, the SEC took a lead role in rewriting industry regulation via the Dodd-Frank Wall Street Reform and Consumer Protection Act, and has moved to expand its oversight powers into derivatives and over hedge funds. The SEC also launched a whistle-blower program to encourage and reward informants on major violations of securities laws. The SEC said that under Schapiro, its enforcement activity has rapidly increased to hit 734 actions in fiscal 2012. Last week the SEC brought its largest insider trading suit to date, implicating the giant hedge fund manager SAC Capital in charges that one of SAC\'s managers had obtained secret reports on drug tests by two pharmaceutical companies to rack up $276 million in illicit trading gains. Under Schapiro the SEC has moved to better oversee program trading that has sparked \"flash crashes\" in stocks and the markets. Richard Ketchum, chief executive of FINRA, praised Schapiro\'s tenure. \"Chairman Schapiro took over the SEC at an extremely challenging time, both for the agency and for the markets,\" he said. \"She significantly improved the Commission\'s enforcement division, boosted agency morale and streamlined the Commission\'s processes.\" Duncan Niederauer, chief executive of NYSE Euronext, which operates the New York Stock Exchange, meanwhile said that Walter \"brings a wealth of regulatory and financial markets expertise that makes her a strong choice to lead the SEC.\"