The human transporters. The figure shows the major families of membrane proteins that primarily function as transporters, proteins that facilitate the movement of substrates across a membrane. The tree structures represent three major classes of transporters that differ in energy dependence and utilization during transport. The number of genes and function for each family have been determined by combining results from clustering, using the ISODATA algorithm, with data from the literature and public databases. Primarily TCDB and HGNC were used. Consensus TM helix numbers have been set through evaluation of data from external resources among the literature and databases together with prediction results from Phobius, SOSUI, and TMHMM. The Other table contains transporter families that do not fit into any of the larger families. The table of Auxiliary transport proteins contains families that are not essential for transport, but modulate other transport proteins through different interactions.