"CSI" says goodbye to one of its own this week when series star Marg Helgenberger leaves the Vegas crime lab after a stellar 12-season run. But she's not the first star to leave a hit show. (Heck, she's not even the first to leave "CSI"; co-star William Petersen left the cast in 2009.) Marg's exit inspired us to look back at 15 of the most memorable star departures from top-rated TV series.
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