Marg left the show in season 12 when Willows decided to leave CSI for the FBI. In the first episode of the new series, Sara explains Catherine's whereabouts to Brass: “Catherine has rolled out a red carpet, she would be here, you know that, if she could.” Brass replies, “Yeah, she's got a little grandbaby in Dublin.”

Why did Marge Helgenberger leave CSI?

After nearly four years, Helgenberger left the show in January 1986 to pursue new opportunities.

Why did Jorja Fox leave and return to CSI?

On October 15, 2007, Fox told Entertainment Weekly that she left CSI, saying that she wanted a break "from the commitment of a weekly television series." Fox requested that the money collected during the "Dollars for Sense" campaign be donated to CASA, an organization dedicated to aiding foster children.