FAQ: Viral Gene Delivery

Q: How do I determine which viral vector to use for gene delivery?

A: There are several considerations when deciding on the best viral vector:

  • Do I need transient or stable gene expression?
  • Do I need to infect dividing or non-dividing cells?
  • How important is potential immune response from my target cell?
  • Will you be using your viral vector in vivo or in vitro?

You can find a quick comparison of viral vectors with respect to these and other factors in our Selection Guide.

Q: Measuring the titer of my virus is too complex and takes too long. Why should I bother?

A: Effective gene delivery depends on using the optimal MOI (multiplicity of infection) for your target cell, and knowing the titer of your virus is essential to using the correct MOI. Also, because viral gene delivery is a complex, multi-step process, occasionally you don’t get the gene expression you expect. Knowing your viral titer will help determine where the problem might be: during viral packaging, during purification, or during transduction of your target cell. While it’s true that most traditional methods of measuring viral titer are tedious and time-consuming, we offer convenient, user-friendly methods to determine viral titer for AAV, adenovirus, lentivirus and retrovirus.

Q: I have seen viral titer expressed in various formats: VP/mL, TU/mL, ifu/mL and pfu/mL? What is the difference?

A: There are two main types of viral titer:

  1. Physical titer is strictly a measurement of how much virus is present, and is usually expressed as the number of viral particles per mL (VP/mL).
  2. Functional titer, also known as infectious titer, is the measurement of how much virus actually infects a target cell. Functional titer may be expressed in the form of transduction units per mL (TU/mL), plaque-forming units per mL (pfu/mL), or infectious units per mL (ifu/mL), depending on the viral vector.

Functional titer is always a more accurate measurement because it measures how much virus actually gets into the target cell. However, functional titer usually takes much longer to determine and is sometimes not practical. In such cases physical titer may be measured, and functional titer may be calculated from physical titer. Functional titer will always be lower than physical titer, usually by a factor of 10 to 100-fold.


Q: What is the difference between purified and unpurified viral supernatant? Can filtering the supernatant with a low protein-binding filter be considered purification?

A: Purified viral supernatant means that it is free of serum protein and contaminants. Purification usually involves column chromatography or ultracentrifugation. Supernatant passed through a protein-binding filter simply removes cell debris and large insoluble particles. Such a supernatant would not be considered purified and would be particularly unsuitable for any in vivo use.