<Tr> <Th> Occupation </Th> <Td> Actor </Td> </Tr> <Tr> <Th> Years active </Th> <Td> 1987--present </Td> </Tr> <P> John Mark Galecki (born April 30, 1975) is an American actor . He is known for playing David Healy in the ABC sitcom Roseanne from 1992--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). </P> <P> 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 . </P>

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