Growth kinetics of rLaSota and rLaSota expressing different forms of IBV S protein of Egyptian IBV variant strain EG/CU/4/2014. DF-1 cells were infected with the indicated viruses at MOI of 0.01. Normal fresh chicken embryo allantoic fluid was added to the medium as exogenous source of protease. Samples of the supernatant media were collected at 12 h intervals and the viral titers were determined by limiting dilution on DF-1 cells. Means and SEM of two independent experiments are plotted at each time point.