Fig. 9. The deletion mutant IFN(1-123) is not internalized compared to wild-type IFN. Internalization assays were run as described previously (Farrar et al., 1991). Briefly, 125I-labeled wild-type and mutant IFN (80-85 µCi/µg) were added to WISH cells at 5 nM. Cells were incubated at 4°C for 90 minutes, washed at 4°C and then incubated at 37°C for the indicated times. At the indicated times internalized cpm was counted from acid-washed cells lysed in 1% SDS. Data are plotted as a percentage of the total ligand bound that was internalized. Samples were run in triplicate and data are plotted as mean±s.d.