The cluster of differentiation 81 (CD81),a member of the transmembrane 4 superfamily,is primarily found to be expressed in a wide variety of cells including T and B cells of vertebrates as a critical modulator.In the present study,the open reading frame of a CD81 gene homolog (Lja-CD81)was cloned in lamprey,Lampetra japonica,which is 702 bp long and encodes a protein of 233-amino acids.Although Lja-CD81 seems to be close to CD9 molecules in their full-length sequences,Lja-CD81 possesses higher identity to vertebrates’ CD81 than to CD9 (including a lamprey CD9) molecules in their large extracellular loops.In addition,it also possesses a myristoylation site (Met-Gly-Val-Glu-Gly-Cys-Leu-Lys) in its N-terminal region which is identical to the N-terminal regions of CD81 molecules.These data suggest that CD9 and CD81 molecules diverged no later than the emergence of jawless vertebrates.The mRNA levels of Lja-CD81 in lymphocytes and supraneural myeloid bodies were up-regulated significantly after stimulation with mixed antigens,and a similar expressional patt of Lja-CD81 at protein level was also confirmed.Furthermore,Lja-CD81 was found to be co-localized with variable lymphocyte receptor B (VLRB) evenly on the cell membrane of peripheral blood lymphocytes isolated from control group,but they were found to aggregate on one side of the membrane of peripheral blood VLRB+ lymphocytes after stimulation with mixed antigens.All these results indicate that the Lja-CD81 identified in lamprey may play an important role in the immune response of lamprey VLRB+ lymphocytes.