Any of a large family of proteins that catalyse the phosphorylation of a tyrosine residue of a protein by ATP. They are components of numerous signalling pathways inside cells, notably ones regulating cell growth and differentiation. There are two main types. Receptor tyrosine kinases are cell surface receptors having intrinsic tyrosine kinase activity. There are around 20 classes in humans; examples include the insulin receptor, fibroblast growth factor receptors, neurotrophin receptors, and epidermal growth factor receptors. Nonreceptor tyrosine kinases, such as Src tyrosine kinases, occur wholly in the cytosol but can associate with surface receptors to activate intracellular signal pathways.