The human receptors. The figure shows the major families of membrane proteins that are classified to primarily function as receptors, proteins that trigger a cellular response upon binding of specific ligands. The tree structures give a comprehensive view of subfamilies in the largest families. 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 the Human Plasma Membrane Receptome database and HGNC have been 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 receptor families that do not fit into any of the larger receptor families.