<p>Orotidine 5'-phosphate decarboxylase (OMPdecase) [<cite idref="PUB00003724"/>, <cite idref="PUB00002768"/>] catalyses the last step in the <i>de novo</i> biosynthesis of pyrimidines, the decarboxylation of OMP into UMP. In higher eukaryotes OMPdecase is part, with orotate phosphoribosyltransferase, of a bifunctional enzyme, while the prokaryotic and fungal OMPdecases are monofunctional protein.</p><p>Some parts of the sequence of OMPdecase are well conserved across species. The best conserved region is located in the N-terminal half of OMPdecases and is centred around a lysine residue which is essential for the catalytic function of the enzyme.</p>