As serious side effects affected recent virus-mediated gene transfer studies, novel vectors with improved safety profiles are urgently needed. In the present study, replicationdeficient retroviral vectors based on feline foamy virus (FFV) were constructed and analyzed. The novel FFV vectors are devoid of almost the complete env gene plus the internal promoter -accessory bel gene cassette including the gene for the viral transcriptional transactivator Bel1/Tas. In these Bel1/Tas-independent vectors, expression of the lacZ (b-galactosidase) marker gene is directed by the heterologous, constitutively active human ubiquitin C promoter (ubi). Env-transcomplemented vectors have unconcentrated titers of more than 10 5 transducing units/ml.The vectors allow efficient transduction of a broad array of diverse target cells, which can be increased by repeated vector exposure. However, the number of lacZ marker gene expressing cells decreased slightly upon serial passages of the transduced cells. Vectors carrying a self-inactivating (SIN) deletion of the TATA box and most parts of the viral promoter were not rescued by wt FFV whereas those with the intact or a partially deleted promoter were readily reactivated. This finding indicates that the viral promoters are in fact non-functional, pointing to a highly advantageous safety profile of these new FFV-ubi-lacZ-SIN vectors.