The common gamma chain (gammac) is the central signaling unit for a number of cytokine receptors collectively known as the gammac cytokine receptor family. gammac is critical for ligand binding and signaling by gammac cytokines. gammac cytokine signaling had been thought to be mainly regulated by cytokine-specific receptor alpha chain expression levels with little or no effect by gammac surface levels because gammac expression was presumed to remain unchanged during T-cell activation and development. The extent of gammac cytokine responses is thought to be regulated by cytokine specific receptor subunits and not by the gammac receptor. In contrast to this prevailing view, we have recently reported that gammac itself actively regulates gammac cytokine responses. Interestingly, gammac exerted its regulatory effects not only as a conventional membrane receptor protein but also as a secreted protein whose expression was upregulated upon T-cell stimulation. Here we will review how a soluble form of gammac, which is generated by alternative splicing, regulates gammac cytokine signaling and plays a role in controlling immune activation related to autoimmune disease.