The proportion of a receptor type that is bound by a compound at a given concentration — not the same as biological effect.A compound may produce near-maximum biological response at 10–20% receptor occupancy, because signal amplification downstream multiplies the initial receptor activation. Conversely, a compound with high receptor occupancy may produce little effect if the receptor-effector coupling is inefficient. Receptor reserve, signal amplification, and desensitization all mediate the relationship between receptor occupancy and observed effect.