The gaseous hormone ethylene regulates many aspects of plant growth and development. Ethylene is perceived by a family of high–affinity receptors typified by the ETR1 protein from Arabidopsis. The ETR1 gene codes for a protein, which contains a hydrophobic N–terminal domain that binds ethylene and a C–terminal domain that is related in sequence to histidine kinase–response regulator two–component signal transducers found in bacteria. A structural model for the ethylene binding domain is presented in which a Cu(I) ion is coordinated within membrane–spanning α–helices of the hydrophobic domain. It is proposed that binding of ethylene to the transition metal would induce a conformational change in the sensor domain that would be propagated to the cytoplasmic transmitter domain of the protein. A total of four additional genes that are related in sequence to ETR1 have been identified in Arabidopsis. Specific missense mutations in any one of the five genes leads to ethylene insensitivity in planta. Models for signal transduction that can account for the genetic dominance of these mutations are discussed.