Deen Joseph Castronovo is an American drummer and singer best known for being a member of classic rock band Journey and hard rock acts Bad English and Hardline. He currently plays drums and shares lead vocals for the bands Journey, Generation Radio, and Revolution Saints. He has been a touring and studio player for Ozzy Osbourne, Steve Vai, Paul Rodgers, and Black Sabbath bassist Geezer Butler's GZR.

GZR was an American heavy metal band founded by and named after Black Sabbath bassist/lyricist Geezer Butler. The band has been marketed with three different names on different releases, g//z/r in 1995, geezer in 1997, and GZR in 2005. Most fans refer to the band as "geezer", although Butler himself refers to the band name as "gee-zed-R". This incarnation of the band is not to be confused with Butler's previous attempt in 1985 to form a solo band, then known as the Geezer Butler Band – the two outfits are largely unrelated.