Topology of a transmembrane protein refers to orientations (locations of N- and C-termini) of membrane-spanning segments with respect to the inner or outer sides of the biological membrane occupied by the protein. Several databases provide experimentally determined topologies of membrane proteins. They include Uniprot, TOPDB, OPM, and ExTopoDB. There is also a database of domains located conservatively on a certain side of membranes, TOPDOM. It is also possible to predict beta-barrel membrane proteins' topology.