Star Trek in particular is referred to frequently, and Sheldon identifies strongly with the character of Spock, so much so that when he is given a used napkin signed by Leonard Nimoy as a Christmas gift from Penny, he is overwhelmed with excitement and gratitude ("I possess the DNA of Leonard Nimoy?!"). Star Trek: The Original Series cast members William Shatner and George Takei have made cameos, and Leonard Nimoy made a cameo as the voice of Sheldon's vintage Mr. Spock action figure. Star Trek: The Next Generation cast members Brent Spiner and LeVar Burton have had cameos as themselves, while Wil Wheaton has a recurring role as a fictionalized version of himself. Leonard and Sheldon have had conversations in Klingon.