Protein kinases can become mutated, stuck in the "on" position, and cause unregulated growth of the cell, which is a necessary step for the development of cancer.

[1] Kinase is a large family of enzymes that are responsible for catalyzing the transfer of a phosphoryl group from a nucleoside triphosphate donor, such as ATP, to an acceptor molecule.

[2] Phosphorylation at tyrosine residues controls a wide range of properties in proteins such as enzyme activity, subcellular localization, and interaction between molecules.

[3] Finally mutations can cause some tyrosine kinases to become constitutively active, a nonstop functional state that may contribute to initiation or progression of cancer.

[3] To be specific, Lyn, a type of kinase in the Src family that was identified in the nuclear matrix, appears to control the cell cycle.

[3] It has also been demonstrated that phosphorylation of a middle-T antigen on tyrosine is also associated with cell transformation, a change that is similar to cellular growth or reproduction.

Transmembrane signaling due to receptor tyrosine kinases, according to Bae et al. (2009), relies heavily on interactions, for example, mediated by the SH2 protein domain; it has been determined via experimentation that the SH2 protein domain selectivity is functional in mediating cellular processes involving tyrosine kinase.

[2] The results of some newer research have also indicated that the aforementioned cytokine receptors function with members of the JAK tyrosine kinase family.

[2] Tyrosine residues located in the cytoplasmic domain of the erythropoietin receptor are consequently phosphorylated by the activated protein kinase JAK.

[7] This supports the notion that trafficking, a term for the modification of proteins subsequent to mRNA translation, may be vital to the function of receptor signaling.

Receptor tyrosine kinases play pivotal roles in diverse cellular activities including growth (by signaling neurotrophins), differentiation, metabolism, adhesion, motility, and death.

Binding of a ligand to the extracellular region causes a series of structural rearrangements in the RTK that lead to its enzymatic activation.

In particular, movement of some parts of the kinase domain gives free access to adenosine triphosphate (ATP) and the substrate to the active site.

This triggers a cascade of events through phosphorylation of intracellular proteins that ultimately transmit ("transduce") the extracellular signal to the nucleus, causing changes in gene expression.

In every case, the result is a hyper-active kinase, that confers an aberrant, ligand-independent, non-regulated growth stimulus to the cancer cells.

For example, the T-cell antigen receptor leads to intracellular signalling by activation of Lck and Fyn, two proteins that are structurally similar to Src.

Imatinib (brand names Gleevec and Glivec) is a drug able to bind the catalytic cleft of these tyrosine kinases, inhibiting its activity.

[5] Protein tyrosine kinases that are encoded by the Rous sarcoma virus cause cellular transformation, and are termed oncoproteins.

Much research has already noted the significant effect that inhibitors of the radically functioning protein tyrosine kinase enzymes have on related ailments.

[16] Gefitinib is a tyrosine kinase inhibitor that targets the epidermal growth factor receptor, inducing favorable outcomes in patients with non-small cell lung cancers.

Gefitinib, functioning as an epidermal growth factor receptor tyrosine kinase inhibitor, improved symptoms related to non-small cell lung cancer and resulted in radiographic tumor regressions.

Gefitinib's inhibition of the survival signals may be a contributing factor to its efficacy as a drug for non-small cell cancer treatment.

[16] Gefitinib is well endured by humans, and treatment resulted in a symptom improvement rate of 43% (with 95% confidence in a 33%–53% interval) for patients that received 250 mg of Gefitinib and 35% (with 95% confidence in a 26%–45% interval) for those that received 500 mg.[15] In the trial, epidermal growth factor receptor showed a rapid response to the inhibitor, as demonstrated by the improvement of the cancer symptoms.

In each group, improvements were noted after a single week of epidermal growth factor receptor tyrosine kinase inhibitor treatment.

[15] Gefitinib application once per day caused “rapid” symptom improvement and tumor regressions in non-small cell lung cancer patients.

[15] Also, a death occurred possibly due to epidermal growth factor receptor tyrosine kinase inhibitor treatment; however, the correlation is not exactly clear.

Nevertheless, the side-effects of Gefitinib were only “generally mild, manageable, noncumulative, and reversible.”[15] Unfortunately, ceasing to take the inhibitor may be the only reversal strategy of the unfavorable symptoms.

[17] July 12, 2013 FDA approved afatinib "multiple receptor, irreversible TKI" for the first-line treatment of patients with metastatic non-small cell lung cancer (NSCLC) whose tumors have epidermal growth factor receptor (EGFR) mutation BCR-ABL is a constitutively activated tyrosine kinase that is associated with chronic myeloid leukemia.

[19] Gastrointestinal stromal tumors (GIST) are known to withstand cancer chemotherapy treatment and do not respond to any kind of therapy (in 2001) in advanced cases.

Mutation of c-kit causes the constitutive activity of tyrosine kinase, which results in cancerous gastrointestinal stromal tumors.