The analyses uncovered that two subunits, an NBD and a conserved transmembrane protein, of the metal transporters and the biotin transporters, are related. Functional genomics then uncovered many more transporters of this type. The description in 2009 of ECF systems as a novel group of membrane transporters for many different substrates contradicted the dogma that ABC-type importers strictly depend on extracytoplasmic soluble solutebinding proteins for delivery of substrate and initiation of the transport cycle. Instead, ECF importers contain substrate-specific transmembrane proteins. S units are in most cases single small membrane proteins and have extremely high affinity for their substrates in the low nanomolar or picomolar range. The primary structures of the S components for different substrates are highly diverse. T components are moderately similar transmembrane proteins with strongly conserved amino acid signatures in a cytoplasmic loop. Since the A components contain the typical features of NBDs including the Walker A and B motifs, the LSGGQ signature sequence and the His motif, they are predicted to function as dimers as all ABC ATPases. The module composed of A and T units is called – for historical reasons – the “energy-coupling factor”. Another unprecedented finding was the fact that the ECF module is shared by several highly diverse S components in one subgroup of ECF transporters which are mainly found among gram-positive bacteria and archaea. Subgroup I comprises systems with a dedicated ECF module in gram-negative and gram-positive bacteria and in archaea. Notably, the S components of two bacterial cobalt transporters and the biotin transporter BioMNY of Reversine Aurora Kinase inhibitor Rhodobacter capsulatus, which are members of subgroup I, were shown by in vivo assays to have significant substrate-uptake activity in the absence of their cognate A- and T units. In contrast, analysis of vitamin uptake by subgroup II folate, pantothenate, riboflavin and thiamine transporters suggest that the corresponding S components FolT, PanT, RibU and ThiT do not function as transporters in a solitary state. Many questions regarding physical and functional interactions among the subunits of ECF transporters and their in vivo oligomeric state remain to be answered. Furthermore, the role of the T components is still not understood. Light-scattering experiments with purified subgroup II ECF transporters of L. lactis have revealed that the S, A1, A2 and T subunits mainly exist in a 1:1:1:1 stoichiometry in detergent solution. On the other hand, in vivo fluorescence analyses of the subgroup I biotin transporter of R. capsulatus suggest, that the S unit BioY oligomerizes in the living cell independent of the presence of the A and T components. This finding is indicative of a transporter complex with a higher-order structure in situ. The T components of ECF transporters may function as docking sites for the membrane-spanning S units and the cytoplasmic A units. Recent crystal structure analysis of the L. lactis S unit ThiT combined with sequence comparisons and mutant studies suggest that an Ala-X3-Ala signature in transmembrane helix I of the L. lactis S units is involved in S unit:T unit interactions.