In an effort to search and analyze cell growth and senescence-related genes of the parasitic protist Trichomonas vaginalis, we launched an EST program and isolated a cDNA clone with a length of 910 basepairs from a T.vaginalis cDNA library.The sequence analysis revealed that this cDNA clone contains an open reading frame of 834 bp.The deduced amino acid sequence has 277 residues and is most homologous to the longevity-assurance gene 1(LAG1)proteins.A predicted Lag1 motif and a TLC domain conserved in Lag1 proteins were detected in the amino acid sequence.The secondary structure analysis demonstrated that this protein possesses six predicted transmembrane domains and a membrane retention signal of endoplasmic reticulum.These data suggest that this clone is probably a LAG1 homolog,which may participate in the regulation of ceramide biosynthesis and in determining life span or cell senescence of the protozoan.The genomic DNA of this gene is also cloned and sequenced.The DNA sequence is identical to that of cDNA,suggesting it may be a intronless gene of the protozoan.