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Characters on The Big Bang Theory often use gentle lullaby "Soft Kitty" to calm Sheldon Cooper (Jim Parsons) in his fits of anxiousness. But now the tune is having the opposite effect on the long-running comedy smash, as CBS, Warner Bros. Entertainment and several other media companies are facing a copyright lawsuit over the ditty, The Associated Press reports.

Edith Newlin, a New Hampshire nursery school teacher who died in 2004, published lyrics about a "soft kitty" in 1937. Now her heirs, daughters Ellen Newlin Chase and Margaret Chase Perry, are seeking unspecified damages in their suit, claiming the series has been continuously violating their copyright by using the song in at least eight episodes since March 2008.

The original Newlin lyrics, called "Warm Kitty," open with the line, "Warm kitty, soft kitty, little ball of fur," while the show's version opens, "Soft kitty, warm kitty, little ball of fur." Newlin was not credited, possibly giving viewers the impression that the lyrics were written by someone affiliated with the series, the lawsuit says.

"The Soft Kitty Lyrics are among the best-known and most popular aspects of The Big Bang Theory," the lawsuit reads. "They have become a signature and emblematic feature of the show and a central part of the show's promotion."

The words have also appeared in various Big Bang Theory merchandise, including clothing, wallets, plush toys, cell phone covers and other products; show producers and actors have led audience sing-alongs at three or more Comic-Con conventions since 2010, according to the lawsuit.

Warner Bros. Entertainment and Big Bang Theory producers allegedly decided to use the lyrics in 2007 and sought permission from Florence, Kentucky-based company Willis Music Co., who published "Warm Kitty" in a 1937 book titled Songs for the Nursery School. The suit claims Willis Music authorized the lyrics' usage without receiving permission from Newlin's heirs, despite the book making clear that Newlin was the author and owned the copyright.