A non-receptor tyrosine kinase (nRTK) is a cytosolic enzyme that is responsible for catalysing the transfer of a phosphate group from a nucleoside triphosphate donor, such as ATP, to tyrosine residues in proteins. Non-receptor tyrosine kinases are a subgroup of protein family tyrosine kinases, enzymes that can transfer the phosphate group from ATP to a tyrosine residue of a protein (phosphorylation). These enzymes regulate many cellular functions by switching on or switching off other enzymes in a cell.