The cDNA sequence of CD4 homologue was cloned from the mandarin fish Siniperca chuatsi.It consists of 2 356 bp,encoding 469 amino acids,and this molecule has a typical four Ig-like domains(V1-C1-V2-C2)and shares some fundamental conserved structures,i.e.WTC motif in D2 domain,N-glycosylation site in D4 domian and the CXC motif in the cytoplasmic tail.Unlike avian and mammalian CD4 molecules,the mandarin fish CD4 sequence lacks the Cys pairs in the first Ig-like domain,but has a unique possible disulfide bond in the third domain.These differences suggest that mandarin fish CD4 molecule may have a different structure,which may in turn influence its binding with MHC Ⅱ molecule and probably subsequent T-cell activation.The CD4 was expressed mainly in thymus,and following stimulation with LPS,its significant increase(P<0.05)at mRNA level was observed 8 hours post injection in thymus,spleen and headkidney.The mandarin fish CD4 shared a relatively high level of similarity(57%) with that in sea bass Dicentrarchus labrax,and in the phylogenetic tree,the mandarin CD4 was clustered with that in sea bass.The cloning of CD4 in the mandarin fish may serve as a basis for further functional characterization of fish T lymphocytes.