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Addison Montgomery (Kate Walsh) is introduced as a guest in season 1.
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Callie Torres (Sara Ramirez) is first introduced in season 2.
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Mark Sloan (Eric Dane) and Erica Hahn (Brooke Smith) are first introduced as guest in season 2.
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Mark Sloan and Callie Torres (Eric Dane&Sara Ramirez) are made main characters in season 3.
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Lexie Grey (Chyler Leigh) is introduced as a guest in sason 3.
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Preston Burke (Isaiah Washington) and Addison Montgomery (Kate Walsh) leave during season 3 but do appear as guest in later seasons.
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Owen Hunt (Kevin McKidd) and Arizona Robbins (Jessica Capshaw) first appear in season 5.
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Erica Hahn (Brooke Smith) leaves the show in season 5.
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George O'Malley (T. R. Knight) leaves the show in season 5.
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Teddy Altman (Kim Raver), Jackson Avery (Jesse Williams), April Kepner (Sarah Drew), and Benjamin Warren (Jason George) are all introduced to the show in season 6.
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Izzy Stevens (Katherine Heigl) leavs the show in season 6.
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Addison Montgomery, Teddy Altman, and Lexie Grey all leave the show in season 8.
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Mark Sloan (Eric Dane) leaves the show in season 9.
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Jo Wilson (Camilla Luddington), Stephanie Edwards (Jerrika Hinton), Shane Ross (Gaius Charles), and Leah Murphy (Tessa Ferrer) join the show in season 9.
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Christina Yang (Sandra Oh) leavs the show in season 10.
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Shane Ross (Gaius Charles) and Leah Murphey (Tessa Ferrer) lave the show in season 10.
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Derek Shepherd (Patrick Dempsey) leaves the show in season 11.
