The invention provides a viral self-inactivating (SIN) vector on the basis of the avian sarcoma leukosis virus (ASLV) as well as a split-packaging system comprising in addition to the SIN vector a first helper plasmid serving for the expression of the viral fusion protein gag-pol and a second helper plasmid serving for the expression of the retroviral envelope protein (env). The first and second helper plasmid, for example contained in a packaging cell line or transiently transfected, serve for the generation of non-replicating (RCR-incompetent) viral particles containing RNA having a SIN LTR according to the invention at the 3′ terminus, wherein the RNA can have a therapeutically effective section which e.g. is denoted a transgene. This 3′ SIN LTR contains an extensive deletion of the U3 region which in the course of the reverse transcription is copied into the 5′ LTR. In addition, in the SIN vector all coding regions of ASLV as well as the retroviral splice donor site are removed.