Sequences necessary for transduction of HERV-Kcon, a consensus of the HERV-K(HML-2) family were analysed and found to reside in the leader/gag region. They act in an orientation-dependent way and consist of at least two sites working together. Having defined these sequences, we exploited this information to produce a simple system to investigate to what extent virions of HERV-Kcon, MLV and HIV-1 have the ability to transduce each other's genomes, leading to potential contamination of gene therapy vectors.