The first inhibitor structure, 11a, was inactive in the Human CoV protease binding pocket – however, from there they could identify the differences between the human protease vs the SARS-CoV main protease. The difference was that SARS-CoV has the largest binding pocket while the human corona virus had the smallest. Compared to the enteroviruses, the coronaviruses also had an extra loop or a “lid” to their binding pockets. These were discoveries that were used to design the remaining 10 inhibitors.