Possible modes by which extracellular mutations in hβ_c , indicated by a star in each panel, lead to constitutive receptor activity. (A) The extracellular mutation induces constitutive heterodimerization with a putative γ signalling subunit. (B) The extracellular mutation leads to hβ_c homodimerization. (C) In this model, hβ_c normally associates with an inhibitory membrane-spanning molecule (I); the extracellular mutations disrupt this association and thus lead to activation. For simplicity, only a single β subunit is shown; however, the inhibitory molecule might normally function in suppressing the activity of a low level of spontaneously forming homodimers or heterodimers.