5-methylcytosine (5-mC) in DNA occurs by the covalent addition of a methyl group at the 5-carbon of the cytosine ring by DNA methyltransferases. This process has been well studied and is generally associated with repression of gene expression. It was also observed that in humans, 5-mC occurs in various RNA molecules including tRNAs, rRNAs, mRNAs and non-coding RNAs (ncRNAs). 5-mC seems to be enriched in some classes of ncRNA, but relatively depleted in mRNAs. Levels of 5-mC are variable in animal genomes, ranging from undetectable amounts in some insects to about 0.1-0.45% of total RNA in human cells. The majority (83%) of 5-mC sites were found in mRNAs. Within these transcripts 5-mC appears to be depleted within protein coding sequences but enriched in 5’ and 3’ UTRs. Two different methyltransferases, NSUN2 and Dnmt2 are known to catalyze 5-mC modification in eukaryotic RNA. There has been strong evidence that RNA cytosine methylation affects the regulation of various biological processes such as RNA stability and mRNA translation. Furthermore, loss of 5-mC in vault RNAs causes aberrant processing into Argonaute-associated small RNA fragments that can function as microRNAs. Thus, impaired processing of vault ncRNA may contribute to the etiology of human disorders related to NSun2-defciency.