Myxovirus resistance (Mx) is an interferon (IFN)-induced protein that specifically prevents growth, in vivo and in vitro, of certain classes of virus. We isolated the Mx cDNA from a leukocyte cDNA library obtained from a Japanese flounder infected with Hirame rhabdovirus (HRV). The Mx cDNA was found to contain 2385 bp coding for 620 amino acids. The amino acid sequence had approximately 51 and 78% identities to the sequences of human and rainbow trout Mx cDNAs, respectively. We propose that the amino acid sequence encoded by this cDNA clone contains a tripartite guanosine-5\'-triphosphate (GTP)-binding motif, GXXXSGKS/T, DXXG and T/NKXD, and a signature of the dynamin family, LPRG(S/K)GIVTR, which are conserved in all IFN-induced Mx proteins of vertebrates. In addition, the C-terminal region of the Japanese flounder Mx protein, like those of other Mx proteins, contains a highly conserved leucine zipper motif. Japanese flounder infected with the HRV showed increased expression of Mx mRNA in the leukocytes starting at 48 h and peaking at 72 h. An RT-PCR (reverse transcription-polymerase chain-reaction) analysis of Mx mRNA from a healthy fish showed that the Mx mRNA was predominantly expressed in the kidney, spleen, intestine, brain, peritoneal cavity fluid and gill, and only minimally expressed in leukocytes, liver, muscle and mucus.