Lentiviruses, such as human immunodeficiency virus (HIV), feline immunodeficiency virus (FIV), equine infectious anemia virus (EIAV), and bovine Jembrana disease virus (JDV) are members of the Retroviridae, viruses with enveloped capsids and a plus-stranded RNA genome. Like all retroviruses, the RNA genome of lentivirus is converted to DNA by reverse transcription (RT) after infection and is subsequently stably integrated into the host-cell genome. However, unlike other retroviruses, lentiviruses can infect both slow-and nondividing cells (1 –3 ). To improve safety, U3 and U5 sequences of the long terminal repeats (LTRs) of the lentiviral vector have been modified to generate self-inactivating (SIN) vectors similar to those of the oncoretroviral vectors (4 ,5 ). The aim of this chapter is to introduce a SIN lentiviral vector system and the application of a sensitive Cre/loxP reporter system for vector titer determination.