Nucleoside kinases are enzymes that play a crucial role in cellular metabolism by catalyzing the phosphorylation of nucleosides. This process involves transferring a phosphate group from a donor molecule, usually ATP, to a nucleoside, converting it into a nucleoside monophosphate. This reaction is essential for the synthesis of nucleotides, which are the building blocks of DNA and RNA. These enzymes are important for various biological processes, including DNA replication and repair, as well as RNA synthesis. Nucleoside kinases are also targets for antiviral and anticancer therapies, as inhibiting their activity can disrupt the proliferation of harmful cells.