Figure 2. Cycle threshold (Ct) changes measured by real-time polymerase chain reaction versus days after infection of the indicated cell lines. The cell-free sample had an initial Ct of 31, which rose to 40 by day 11. Reductions in the Ct or flat-line Ct values (e.g., COS cells) indicate replication of the virus. Continued increases in Ct above the initial value of 31 by days 7 and 11 indicate failure to replicate.