For single-virus experiments, pseudoviruses were labeled with the fluorescent membrane label Texas Red-DHPE (Invitrogen) following the protocol we have previously used for influenza and Zika viruses (48 (link), 58 (link)). Texas Red solution (0.74 mg/mL) in ethanol was mixed with HM buffer (20 mM HEPES, 20 mM MES, 130 mM NaCl [pH 7.4]) at a ratio of 1:40. Fifty microliters of 100-fold-concentrated purified pseudovirus particles (total viral protein concentration of ~2 to 2.5 mg/mL for MLV pseudoviruses as measured via a BCA assay) was mixed with 200 μL of Texas Red-DHPE/HB (HEPES 20 mM, 150 mM NaCl [pH 7.2]) buffer suspension and incubated at room temperature in the dark for 2 h on a rocker. After that, 2.75 mL of HB buffer was added to the mixture, which was divided into two aliquots. Each aliquot was pelleted at 20,000 × g for 60 min at 4°C, resuspended in 25 μL of HM buffer at pH 7.4, stored at 4°C, and used within 1 week.