A press release announcing "Unraveling the Mystery: A Big Bang Farewell "said the 30-minute special will feature memorable scenes and cast interviews, as well as Galecki and Cuoco "sharing some of the best-kept backstage secrets and personal memories from the past 12 years."
The cast taped its 279th and final episode on Tuesday.
Helberg tweeted a photo of him hugging his co-stars.
"We have done this 278 times. Before every show. For 12 years. Tonight we'll do it for the last time," he captioned the image.
Jim Parsons posted on Instagram a photo of his character Sheldon's apartment door in honor of the milestone and wrote: "Thank you, apartment 4A, for being a home to so many dreams come true, to so many friendships made.
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"And thank YOU, ALL of you - yes, YOU, reading this right NOW! As we get ready to tape our final episode tonight, to walk in and out of this apartment door for the last time, it is hard to find the words to articulate what a profound experience this has been. But the words 'love' and 'gratitude' come to mind."
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