Lentiviral vectors for gene therapy stably integrate into the genome of the host but also may inadvertently activate neighboring genes or disrupt splicing patterns. In an effort to design safer vectors, Cesana et al. identified sequences in the viral genome that influence splicing between the integrated viral genome and neighboring host genomic DNA. These sequences included known viral splice donor and acceptor sites as well as numerous splice sites described for the first time in this study. Mutating these splice sites reduced aberrant splicing, although it also reduced viral production. In addition, the authors also found more aberrant splice events when using a vector with a strong viral promoter than a vector lacking a viral promoter. Given the tendency of lentiviral vectors to integrate into actively transcribed genes, strategies to reduce interference with the host genes, such as silencing cryptic splice sites, are crucial to the development of safer vectors for gene therapy. (J. Clin. Invest. 122, 1667–1676, 2012)