Western blot of immunoprecipitates from HEK 293 cells transfected with 1. Mock, 2. IFNAR1 WT, and 3. IFNAR1 S535A and S539A mutants showing specific immunolabeling of the ~110k to ~130k IFNAR1 WT. The immunolabeling is absent in IFNAR1 Ser535 and Ser539 mutants (Control). The immunolabeling is blocked by the phosphopeptide (Phos) used as the antigen but not by the corresponding dephosphopeptide (Dephos).