Abstract: The present invention relates to a method for preparing an expression vector encoding a well-tolerated and highly specific 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 which may be inserted into the genome of a host cell, as well as to the obtained expression vector, cells transfected with these, 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, HIV infection. In particular, the invention relates to well-tolerated and highly specific tailored recombinases capable of combining asymmetric target sequences in a more than 90% of HIV-strains, thereby excising the HIV-1 sequences, and expression vectors encoding them.