Extended Data Fig. shows that ligands with primary amines bind differently to TMPRSS11D and TMPRSS15, depending on the presence of the residue equivalent to S436 in TMPRSS2 (absent in TMPRSS11D but present in TMPRSS15). Thus, we identified a novel amine scaffold ('2222 and '2602) that may be able to discriminate for the presence of S436 in S1.