The RNase P RNA-protein complex located in the nucleus endonucleolytically cleaves near the 5' end of pre-tRNAs, generating the mature 5' end (Ferrari et al. 1980, Bartkiewicz et al. 1989, Jiang et al. 2001, Reiner et al. 2011, reviewed in Jarrous 2002). The site of cleavage is determined by the length of the helices in the acceptor and T stems of the tRNA (Yuan and Altman 1995). Human cells contain distinct nuclear and mitochondrial RNase P activities (Rossmanith et al. 1995), with nuclear RNase P localized in the nucleolus (Jarrous et al. 1999). The nuclear RNase P is similar to bacterial enzymes in having a catalytic RNA component. The mitochondrial RNase P is unusual in containing only protein subunits (Holzmann et al. 2008).