Hormones act through specific receptors on target cells. Hormone receptors are large proteins that determine the specificity of hormone action. Hormones can act by changing membrane permeability, binding to intracellular receptors to affect gene expression, or activating second messengers through G-protein coupling which then mediate the hormone's physiological response. Hormone action can also occur via tyrosine kinase activation either through receptors with intrinsic tyrosine kinase activity or receptors that activate tyrosine kinases.