The monoclonal antibody OVTL3 has a highly restricted reactivity with ovarian carcinomas and defines a surface glycoprotein, OA3, which has been used for immunotargeting. To understand why OA3 is found on ovarian tumors we isolated a complementary DNA by expression cloning. The clone encodes a 323-amino acid protein with 5 putative membrane spanning domains, reminiscent of a membrane receptor or channel, but of a new type with little or no sequence similarity with these families of proteins. Interestingly, the OA3 sequence is highly related to a vaccinia virus encoded protein (VA38) and its extracellular domain is a member of the immunoglobulin V region superfamily.

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