The present invention relates to retroviral vectors carrying sequences encoding naturally occuring, antimicrobial peptides or derivatives thereof for the treatment of mammalian tumours, viral infections such as HIV infection and bacterial and fungal infections. In particular the present invention relates to retroviral vectors which undergo promoter conversion (Procon vectors) carrying such sequences. Since these vectors also carry tumour or virus specific regulatory elements, the therapeutic antimicrobial peptide will be delivered and expressed only in relevant, affected cells and not in innocent bystander cells.