Music director and conductor Michael Tilson Thomas. This month he took over the San Francisco Symphony. He's been conducting for 25 years. At the age of 24 years old he was appointed assistant conductor of the Boston Symphony, and began filling in regularly as conductor when the musical director's health began to fail. THOMAS was mentored by Leonard Bernstein. However, Thomas's roots are in the theater: His grandparents were Boris and Bessie Thomashefsky, who founded the Yiddish theater in New York. There's a new book about him, Michael Tilson Thomas: Viva Voce.