The newly characterized sperm-specific Na^(+)/H^(+) exchanger stands out by its unique tripartite domain composition^(1,2). It unites a classical solute carrier unit with regulatory domains usually found in ion channels, namely, a voltage-sensing domain and a cyclic-nucleotide binding domain^(1,3), which makes it a mechanistic chimera and a secondary-active transporter activated strictly by membrane voltage. Our structures of the sea urchin SpSLC9C1 in the absence and presence of ligands …