The gaseous phytohormone ethylene plays a very important role in regulating plant dormancy, growth, and development. An ethylene signal is perceived and responded to by an ethylene receptor protein family in Arabidopsis, and several genes from this family (ETR1, ERS, ETRl, ERS2 and EIN4) have been cloned. By utilizing Arabidopsis ETR\ gene as a molecular probe, a Citrus homologue was initially isolated from a Citrus (Citrus sinensis [L.] Osbeck cv. Valencia) genomic library and used to screen a cDNA library prepared from the same plant. The full length of the cDNA is 2400 bp with an open reading frame of 634 amino acid residues. The gene shares 69.1% nucleotide homology with ETR1, 73.3% with ERS, 72.4% with RP-ERS\, and 71.4% with NR of tomato. Based on sequence alignment analysis, Citrus-ERS appears to be a homologue of ERS. Both ERS and the Citrus-ERS homologues have an N-terminal ethylene-sensor domain and a histidine kinase domain, but lack a responsor domain found in the C-terminal of ETRl. The four amino acids that confer dominant ethylene insensitivity in the A. thaliana ETRl mutants are conserved.