Helicases are complex and structurally dynamic enzymes that couple ATP (or other nucleotides) hydrolysis at one site with RNA or DNA duplex unwinding at another. Given that the fragments in the targeted Nsp13 structure (PDB codes 5RLH, 5RLZ, 5RML, and 5RMM) bound to full-length Nsp13 in the absence of ATP or RNA , a similar form of the protein was used in a surface plasmon resonance (SPR) assay to measure the direct binding of the 1957 Round 1 compounds to the full-length protein (Table ). Nsp13 is a core component of the replication-transcription complex that also includes the viral RNA-dependent RNA polymerase (RdRp) , but the isolated protein was used in the assay for two reasons: first, fragments in the PDB were bound to the isolated monomer, and second, binding to RdRp would have obscured the results. All compounds were also tested in an ATPase assay (Table ), but we saw no correlation between SPR and ATPase assays and decided to rely on direct binding (SPR) to advance compounds to Round 2. Indeed, false positives in the ATPase assay that may bind to other assay-specific molecular components should be true negative in SPR, while true positives binding the RNA site in the SPR assay may not inhibit the ATPase activity. We also cannot discount the possibility that SPR hits may bind at unexpected and functionally neutral sites.