How to estimate viral titre? - (Mar/10/2010 )

I packaged lentiviruses to deliver pGIPZ shRNA plasmids (with GFP) (Open Biosystems). According to the protocol from Open Biosystems, I can estimate the viral titre by transducing 293T cells (with serial dilution) and score the number of GFP positive colonies. However, I was wondering how one can estimate the viral titre if the shRNA plasmid does not have GFP. Any ideas?

Let's say you have already titered your GFP virus. Then you can use the GFP virus as your standard and transduce cells with the lentivirus without GFP marker side by side, isolate DNA and perform real-time to compare the lentiviral cDNA copy numbers, convert the copy number to titer.