"The TC-DB website details a comprehensive classification system for membrane transport proteins known as the Transport Classification (TC) system. The TC system is analogous to the Enzyme Commission (EC) system for classification of enzymes, except that it incorporates both functional and phylogenetic information. Descriptions, TC numbers, and examples of over 600 families of transport proteins are provided. Transport systems are classified on the basis of five criteria, and each of these criteria corresponds to one of the five numbers or letters within the TC# for a particular type of transporter. Thus a TC# normally has five components as follows: V.W.X.Y.Z. V (a number) corresponds to the transporter class (i.e., channel, carrier (porter), primary active transporter or group translocator); W (a lettter) corresponds to the transporter subclass which in the case of primary active transporters refers to the energy source used to drive transport; X (a number) corresponds to the transporter family (sometimes actually a superfamily); Y (a number) corresponds to the subfamily in which a transporter is found, and Z corresponds to the substrate or range of substrates transported. Any two transport systems in the same subfamily of a transporter family that transport the same substrate(s) are given the same TC#, regardless of whether they are orthologues (e.g., arose in distinct organisms by speciation) or paralogues (e.g., arose within a single organism by gene duplication). Sequenced homologues of unknown function are not normally assigned a TC# unless they represent a unique (sub)family or are from an unrepresented organismal kingdom. If multiple dissimilar subunits are present, they are numbered S1, S2, S3...Sn. Classification categories 8 and 9 are reserved for accessory transport proteins and incompletely characterized (families of) transporters, respectively. Mechanisms of bulk transport via endocytosis, exocytosis and intracellular trafficking, observed in eukaryotes and prokaryotes (Lonhienne et al., 2010) are not included in TCDB."