RNase MRP (also called RMRP) is an enzymatically active ribonucleoprotein with two distinct roles in eukaryotes. RNAse MRP stands for RNAse for mitochondrial RNA processing. In mitochondria it plays a direct role in the initiation of mitochondrial DNA replication. In the nucleus it is involved in precursor rRNA processing, where it cleaves the internal transcribed spacer 1 between 18S and 5.8S rRNAs. Despite distinct functions, RNase MRP has been shown to be evolutionarily related to RNase P. Like eukaryotic RNase P, RNase MRP is not catalytically active without associated protein subunits. Mutations in the RNA component of RNase MRP cause cartilage–hair hypoplasia, a pleiotropic human disease. Responsible for this disease is a mutation in the RNase MRP RNA gene (RMRP), a non-coding RNA gene. RMRP was the first non-coding nuclear RNA gene found to cause disease. Mechanism and mutation effects RNase MRP and its role in pre-rRNA processing has been previously studied in Yeast cells. RNase MRP has been shown to cleave an internal transcribed spacer, specifically ITS1 at the specific site A3 of the rRNA precursor, leading, after additional trimming, to the formation of the mature 5′-end of 5.8S rRNA. Recent data that has been gathered using several temperature-sensitive RNase MRP mutants that showed that inactivation of RNase MRP leading to severe reduction of the abundance of all early intermediates in the typical rRNA processing pathway. However, the transcription of the rRNA precursor is not affected, thus suggesting that RNase MRP plays a key role in the processing of rRNA beyond the cleavage of the A3 site in ITS1. Further research in Yeast cell RNase MRP has shown a potential role in the regulation of the cell cycle. RNase MRP mutations led to missegregation of plasmids and caused cell cycle delay at the end of mitosis, followed by a buildup of cyclin B2 (CLB2) protein (resulting from increased CLB2 mRNA concentration that codes for the CLB2 protein). RNase MRP also demonstrated cleavage ability of the 5′-UTR of CLB2 mRNA that allows for rapid 5′-to-3′ degradation by XRN1, an exoribonuclease enzyme. Link to RNAse P RNase P and RNAse MRP are ribonucleoprotein complexes that are important in RNA processing. Both subunits have a highly conserved P4 helical region, which is a type of nucleic acid tertiary structure. This region is needed for catalytic function, and is probably an important part of the enzyme's active site. RNAse P is found in