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Marcus is an accomplished actor and musician living in Los Angeles. He is best known for the role of Peter Gray, on the long running CBS Drama, "Judging Amy". Recently, Marcus was seen in the recurring role of Jeff, on the HBO series Tell Me You Love Me. Amongst his other, numerous TV appearances, including Soaps and M.o.w.'s: Flying Blind (Fox Series Regular), The Mentalist, Medium, House, CSI, Monk, Cold Case, Crossing Jordan, Criminal Minds, Life, The X Files, Cosby, Homicide: Life On The Streets, and Quantum Leap.

Film work includes: Hamlet, Necessary Roughness, Business of Strangers, Mr. And Mrs. Bridge, Judy Berlin, On The Doll, Three Ninjas Kick Back and The Chosen One.

Marcus' extensive theater credits include work on Broadway in Horton Foote's "The Young Man From Atlanta" and at Lincoln Center, in Shakespeare's " Measure For Measure". Off-Broadway he has appeared at The Manhattan Theater Club, The Lambs and The WPA, amongst others. Regionally, Marcus has worked all over the country, from The Seattle Rep, to The Guthrie, The Goodman, Hartford Stage, Long Wharf, Williamstown (over 20m productions), The Yale Rep and The Eugene O'neil Theater Center.

Marcus is a graduate of The Yale School Of Drama and Bowdoin College.

Marcus is the son of former Major League Baseball Commissioner Bart Giamatti, and the brother of actor Paul Giamatti.

He is an avid, life long Boston Red Sox fan.