Retroviral vectors are commonly used in ex vivo gene therapy protocols. The structure of vectors basically consists of one gene of interest and a selectable marker gene. Fast selection without damaging cells is a critical step for ex vivo gene therapy protocols. Blasticidin S deaminase isolated from Bacillus cereus has a neutralizing action on the highly toxic antibiotic blasticidin S (BS). A commercially available gene coding for blasticidin S deaminase (bsr) when used to construct retroviral vectors, LBSN and LNSB, provided very low levels of BS deaminase activity, precluding their routine use in gene transfer experiments. However, with the introduction of specific mutations into the bsr gene based on the Kozak consensus sequences and deletion of a 5' untranslated sequence to generate bsrm, we were able to construct a retroviral vector encoding resistance to high doses of BS (at least 16-fold above the usual lethal dose in NIH3T3 cells), showing that bsrm/BS may provide a useful system for selection of transduced mammalian cells.