"Star Trek" in particular is frequently referenced, 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?!"). ' 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. ' 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 the Klingon language.