RNase MRP cleaves mtRNA transcripts that are complementary to the leading-strand origin of mammalian mtDNA, and therefore, it is a candidate for playing a role in primer RNA metabolism. The predominant site of cleavage on single-strand mtRNA is one that may occur in vivo as well, given the fact that transcript termini map at this position. RNase MRP could produce 3 -OH groups for extension by mtDNA polymerase or could be somehow involved in stabilizing primers at the 5 end or in 5 -end editing of transcripts new evidence is consistent with any or all of these roles (see section below on regulation).