Retroviral vectors are commonly used for gene expression in vitro and in vivo. The structure of these vectors usually consists of one gene of interest and a selectable marker gene. Fast selection without damaging cells is a critical step for significant gene expression. Blasticidin S deaminase (Bsd) isolated from Bacillus cereus has a neutralizing action on the highly toxic antibiotic blasticidin. A variety of commercially available retroviral vectors, such lentiviral expression systems, utilize Bsd expression for selection using blasticidin. This selectable marker allows selection of stable mammalian cell lines in as little as one week.