Exhibit 99.1 Page 9 of 11 Pages SCHEDULE 13D CUSIP No. 59156R108 JOINT FILING AGREEMENT Pursuant to Rule 13d-1(k)(1)(iii) promulgated under the Securities Exchange Act of 1934, as amended, each of the undersigned agrees that the Statement on Amendment No. 53 to Schedule 13D, to which this exhibit is attached, is filed on his or her behalf. Dated: May 7, 2013 * ---------------------------- Cheryl W. Grise * ---------------------------- Carlos M. Gutierrez * ---------------------------- R. Glenn Hubbard * ---------------------------- Steven A. Kandarian * ---------------------------- John M. Keane * ---------------------------- Alfred F. Kelly, Jr. * ---------------------------- James M. Kilts * ---------------------------- Catherine R. Kinney Page 10 of 11 Pages SCHEDULE 13D CUSIP No. 59156R108 * ---------------------------- Hugh B. Price * ---------------------------- David Satcher, M.D. * ---------------------------- Kenton J. Sicchitano * ---------------------------- Lulu C. Wang * By /s/ Ricardo A. Anzaldua ----------------------------- Ricardo A. Anzaldua Attorney-in-fact