Apo sites. Using the list of Uniprot accession codes referenced to the covPDB proteins, a list 3,527 apo counterparts of the covPDBs (apo-covPDBs) was initially scraped from the RCSB database while filtering out entities with non-polymer molecules (except waters, metal ions, and small-molecules typically used in buffers). The refined list of apo-CovPBDs used for analysis was arduously compiled by manual alignment to the covPDB counterparts and crosschecking for presence of all pocket-forming residues (apo-covPDBs, SupplementaryFile1).