293T cells were cultured in 10-cm cell culture dishes (3×106 cells/dish). The lentiviral vector packaging system was made as follows: a solution of 500 μl was first prepared consisting of 12 μg plasmid pCMV-Δ8.2, 10 μg pCMV-VSV-G, 22 μg transfer expression plasmid lenti-GSN, and 125 μl 2 mM CaCl2 in deionized distilled water. CaCl2/DNA was then added dropwise while vortexing to a volume of 2×HEPES-buffered saline (HBS) to a total of 1 ml, and was added to the cells at a density of 80%. GFP expression was observed by fluorescent microscopy after 24 h. The supernatant was harvested by centrifugation at 3000 rpm for 5 min at 4°C after 48 h and the ratio of positive cells was measured by using FACSCalibur flow cytometer (Becton Dickinson, Franklin Lakes, NJ, USA). The high-concentration lentiviral concentrate was used to infect the YTS cells.