Described is a polynucleotide molecule being a fragment of a polynucleotide sequence of an infectious human endogenous retrovirus, said polynucleotide molecule comprising a sequence selected from the group consisting of (a) a sequence with at least 98%, preferably 99%, still preferred 99.5%, identity to a sequence according to SEQ ID No 1, (b) a sequence which hybridises under stringent conditions with a nucleotide sequence according to said SEQ ID No 1, (c) a sequence which differs from said sequences (a) or (b) due to degeneration of the genetic code and (d) a sequence comprising sequences inserted into vectors pCR4-Topo and deposited as "MERV-env", "MERV-gag", "MERV-prt" and "MERV-pol" at the DSMZ on 26 Sep. 2001 or fragments thereof.