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1. Pfu:

The plaque-forming unit (pfu) is the number of viral particles’ capability to form the plaques (the area of cell lysis by per viral    particle).  It is a functional measurement rather than a measurement of the absolute quantity of particles: viral particles that are defective or which fail to infect their target cells will not produce a plaque and thus it will not be counted. For example, a solution of adenovirus with the concentration of 1,000pfu/µl indicates that there are 1,000 infectious virus particles in one microliter of the solution. In case of adenovirus, the procedure for quantified pfu takes 3 weeks in HEK 293 cells.

2. Ifu (infectious unit):

Ifu can also be used instead of pfu. Ideally, both give the same number. Current popular method to estimate this number is to count the HEK 293 cells number expressing polyhedron protein (adenovirus envelop) after infection.

3. MOI (Multiplicity of Infection):

It is the ratio of number of virus to the number of the cell. An MOI of 10 indicates that there are 10 transduction units (functional virus, pfu or ifu) for every cell in the well. It is important to note that different cell types require different MOIs for successful infection. In addition, because actual transduction (infection) occurs through statistical processes, even in completely permissive cells like HEK 293 cells, MOI =1 produces only 63% infection, and MOI=5 is required for 99% transduction.

4. Estimated Pfu or Ifu from OD260:

Using the virus prepared by a traditional centrifugation method, Challberg and Ketner reported that pfu can be estimated directly from  OD260 by the formula pfu/ml = OD260 × dilution factor × 3.5 × 109. However, adenovirus prepared by our kit has much higher quality than those and the formula determined empirically is close to OD260 × dilution factor × 2 × 1010 (pfu/ml).

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