Growth kinetics of recombinant viruses. MRC5 cells were infected at a multiplicity of 0.05 PFU/cell with the FIXwt or each recombinant containing (A) a large substitution in the ULb′ region or (B) a substitution in an individual ORF within the FIX(ur)sub2 region. Lysates were collected over a time course and virus yields determined by TCID₅₀ assays on MRC5 cells. The standard error is shown for 3 to 4 independent experiments.