The present disclosure relates to an endogenous Retrovirus K protein (ERVK) with an alternative envelope protein titled CTXLP. Said CTXLP peptide is represented by the sequences set forth in SEQ ID NO: 1. Additionally, antibodies that specifically recognize the epitope(s) set forth in SEQ ID NO:1 are and methods of use thereof and kits comprising the peptide set forth in SEQ ID NO:1 are also included in the present disclosure.