Methylation is the transfer of a methyl group from a methyl donor to a substrate, and such reactions are catalyzed by enzymes called methyltransferases (MTases). The methyl donor for cellular methylation reactions is most commonly S-adenosyl-methionine (SAM), and the substrates include DNA, RNA, lipid, protein, as well as small metabolites. Protein methylation, particularly in the context of histone tail methylation, has been extensively studied because of its ability to regulate gene expression and chromatin structure. However, several studies on non-histone methylation have emerged, and it has been shown that the methylation can also play an important role in the regulation of non-histone protein function. The most recent bioinformatics study reveals that there are 208 proteins categorized as methyltransferase in human, and the majority of these are hitherto uncharacterized (Petrossian and Clarke 2011). Although the number of newly characterized methyltransferases has increased recently, most of the enzymes remain uncharacterized. This thesis focuses on a previously uncharacterized putative methyltransferase, by us denoted as human methyltransferase X (hMTX), belonging to the so-called seven-beta-strand (7BS) superfamily (Petrossian and Clarke 2011). The aim of this study is to characterize the hMTX enzyme using several techniques, including bioinformatics analysis, gene cloning and mutagenesis, overexpression and purification of recombinant enzyme and in vitro methyltransferase assays. In summary, the data presented in this thesis describe the enzymatic activity of a novel human methyltransferase. Moreover, the evolutionary conservation of the enzyme in question (hMTX) suggests that the corresponding reaction occurs in most multicellular organisms. Reference: Petrossian, T. C. and S. G. Clarke (2011). "Uncovering the human methyltransferasome." Mol Cell Proteomics 10(1): M110.000976.