MUC1-C expression and localization was confirmed by confocal imaging as described previously (35 (link), 36 (link)). Briefly, Calu-3 cells were cultured for 24 h on poly-L-lysine-coated glass cover slips in 12-well culture plates. The cells were washed with PBS and infected with SARS-CoV-2 at an MOI of 0.5. After 1 h, viral supernatants were removed and replaced with 2 ml of DMEM containing 2% FBS. The cells were incubated for 45 h at 37°C in a CO2 incubator prior to treatment with PBS or Leptomycin B (LMB, Cell Signaling Technology, 20 nM), an inhibitor of chromosomal region maintenance 1 (CRM1) that is a nuclear export protein. After a 3 h treatment, the cells were prepared for immunofluorescent imaging as follows. Cells were fixed with 4% paraformaldehyde, permeabilized with 0.1% Triton X-100, blocked with 3% BSA, and incubated with rabbit anti-MUC1-CT antibody for 2 h at room temperature. After washing the cells with PBST (0.1% Triton X-100 in PBS) containing 1% BSA, the cells were incubated with Alexa Fluor 488-conjugated secondary antibody (Catalog No. A32790, Thermo Fisher Scientific) for 1 h at room temperature. Nuclei were stained with Hoechst 33258 (Thermo Fisher Scientific). Images were collected using a confocal laser scanning microscope system (CLSM, LSM 710, Carl Zeiss).