While we cannot rule out the possibility that the JSRV Env monoclonal antibody cross-reacts with an endogenous retroviral protein whose expression is upregulated in a subset of human lung tumors, our PCR data support the idea that the antigen detected by the JSRV Env monoclonal antibody is not simply a cross-reacting human protein. Here, ERVW-1 is linked to Neoplasm of the lung.