Subsequently, we performed multi-step virus growth curves in order to further assess the replication of BTV wt and DNS4 in the presence or absence of IFN. CPT-Tert cells were treated with interferon, as described above, and infected at a MOI of 0.01 with wt and mutant viruses. At 24, 48 and 72 h post infection the cell supernatants were collected and the virus titrated in susceptible cells.