In vitro assessment of genotoxicity. (a) 32D cells were transduced with the indicated vectors and plated in IL-3-deficient semisolid medium for 4–5 weeks to allow for colony development. Shown is the mean fold difference with standard deviation as compared with untransduced control cells per 5 × 105 plated cells. CL-SGN, n = 4; LV-SFFVEGFP, n = 34; FVSGW, n = 45; FVSGW-650cHS4-R, n = 43. (b) CD34+ stem cells from cord blood were transduced with either FVSGW or FVSGW-650cHS4-R and cultured in vitro for 5 or 10 days before genomic DNA extraction for modified genomic sequencing-polymerase chain reaction (MGS-PCR). Captured integrations were ordered by position in the genome, and the distances between nearest integration sites were evaluated. Columns represent the percentage of total integrations within 50 kbp of two other integration sites. Each average is based on at least three randomly chosen nonoverlapping unique sets of 1588 integrations from the available MGS-PCR sequencing data. *p < 0.001 compared with LV-SFFVEGFP, FVSGW, and FVSGW-650cHS4-R; **p < 0.05 compared with FVSGW; ***p < 0.01 compared with FVSGW; #p < 0.05 compared with day 5.