How is viral titer determined in VectorBuilder?

After harvesting viral particles, if the viral vector carries a fluorescent reporter gene, we usually first check the quality of virus by transducing the virus into some common cell lines (e.g. 293T or 293A) to observe the expression of fluorescent protein. Different methods are then used to quantify the titer of virus depending on viral type. Occasionally, if there is a major discrepancy between fluorescence observation and quantitative measurement, we will perform re-measurement or additional validation to ensure that viruses manufactured by VectorBuilder are of high quality.