Transient transfection mediated functional characterization of this mutation in HEK 293T cells showed enhanced kinase activity resulting in subsequent strong MEK phosphorylation and activation of ERK1/2 kinases in the absence of TSH [30] suggesting that this is a typical gain-of-function mutation and likely to drive cell proliferation and thyroid carcinogenesis in a kinase dependent fashion. The gene discussed is MAPK3; the disease is thyroiditis.