RNA from P0, SP2, and SP4 infections was analyzed by RT-qPCR for the presence of SARS-CoV-2 gRNA and DI genomes (Fig. 3e [raw Ct values] and Supplementary Fig. 5a [data normalized to GAPDH mRNA levels and expressed relative to CoV-2 gRNA levels]). Here, GAPDH is linked to infection.