Structurally, ENT transporters have eleven transmembrane domains with an intracellular N-terminal and an extracellular C-terminal.
The best characterized ENT transporters are ENT1 and ENT2, which although display a broader range of substrate selectivity, have lower affinities for nucleosides compared to concentrative transporters. They are ubiquitously expressed, for example ENT1 is expressed in erythrocytes, vascular endothelium, placenta, brain, heart, liver and colon. ENT2 displays more or less the same expression pattern but in addition, is strongly expressed in skeletal muscle. ENT3 is a lysosomal pH-dependent transporter capable of transporting adenine, and ENT4 also transports adenine at acidic pH. They are also broadly expressed with ENT3 displaying high expression in the placenta and ENT4 in the heart.