Schematic of combinatorial signaling. A situation where an extra player Y is involved in regulating/interacting with pathway X is considered. The signals X and Y combine to form an active complex with A, but X also participates in a complex with B, as shown in a). This involves the formation of a tertiary complex, AXY. Three distinct mechanisms for the formation of this complex are illustrated in b)-d). In b), A binding to X precedes its binding to Y. In c), A binding to Y precedes its binding to X. In d) X and Y bind to A cooperatively, stabilizing it in its active conformation.