The present invention relates to a method for preparing an expression vector encoding a tailored recombinase, which tailored recombinase is capable of recombining asymmetric target sequences within the long terminal repeat (LTR) of proviral DNA of a plurality of retrovirus strains inserted into the genome of a host cell, as well as to the obtained expression vector, cells transfected with this, expressed recombinase and pharmaceutical compositions comprising the expression vector, cells and/or recombinase. Pharmaceutical compositions are useful, e.g., in treatment and/or prevention of retrovirus infection. In particular, asymmetric target sequences present in a plurality of HIV strains are disclosed, as well as tailored recombinases capable of combining these sequences (Tre 3.0 and 4.0) and expression vectors encoding them.