A cDNA encoding feline CD4 has a unique repeat sequence downstream of the V-like region.

We have cloned, sequenced and expressed a cDNA encoding the feline CD4 glycoprotein. This clone, termed FT121, contains a unique repeat sequence downstream of the V-like region which has not been reported in other mammalian CD4 cDNA. In addition, the cysteine residues at positions 45 and 175 have an unusual pattern. On the other hand, regions other than the unique sequence share significant homology with human CD4 and mouse L3T4 cDNA in the amino acid sequence deduced from the nucleotide sequence, especially in the putative cytoplasmic region. Here, we show that FT121 has a complete open reading frame and efficiently expresses feline CD4 molecules on COS cells. Further we found that the cytoplasmic components, lymphocyte-specific protein-tyrosine kinase p56lck binding sites and phosphorylation site (serine resides), are well conserved, though the extracellular region of the feline CD4 molecule deduced from FT121 is probably different from that of human CD4 and mouse L3T4.