According to nbcnews.com and the Associated Press, Medicare chief Marilyn Tavenner — who oversaw the Obamacare rollout — announced Friday she will step down at the end of February. Tavenner told staff at the Centers for Medicare and Medicaid Services in an e-mail message that she is leaving with "sadness and mixed emotions." Tavenner survived the 2013 technology meltdown of HealthCare.gov, but was embarrassed last fall when she testified to Congress that 7.3 million people were enrolled for coverage. That turned out to be an overcount that exaggerated the total by about 400,000. Calling Tavenner "one of our most esteemed and accomplished colleagues," Health and Human Services Secrerary Sylvia M. Burwell said the decision to leave was Tavenner's. Principal deputy administrator Andy Slavitt will take over as acting administrator.