John Mark Galecki (born April 30, 1975) is known for playing David Healy in the ABC sitcom Roseanne from 1992 to 1997 and Dr. Leonard Hofstadter in the CBS sitcom The Big Bang Theory since 2007. Galecki also appeared in the films National Lampoon's Christmas Vacation (1989), Prancer (1989), Suicide Kings (1997), I Know What You Did Last Summer (1997), Bookies (2003), and In Time (2011). Galecki was born in Bree, Belgium, to American parents of Polish, Irish, and Italian descent. His mother, Mary Louise (Mary Lou) Noon, was a mortgage consultant, and his father, Richard Galecki, was a member of the U.S. Air Force stationed in Belgium, and also worked as a rehabilitation teacher. Galecki is the eldest of three children with a brother, Nick, whom he describes as a "mechanical genius" in the automotive industry, and a sister, Allison. Galecki grew up in Oak Park, Illinois. During an interview with New Zealand radio station ZM, Galecki recalled his childhood relationship with his mother. As a child, he was well known for making up long stories and tales. In such situations, his mother used to make him play the "quiet game", where he had to see how long he could go without talking. He also recalled that despite being a loving mother, she was also very tough. One phrase she would lovingly use was, "I love you, now get out."

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