Seven of the 10 hypotheses were made of five pharmacophoric features while another three had shown four features. The HY-AR was the common feature among all hypotheses. Nine of the 10 hypotheses had Hydrogen-bond acceptor (HBA), three hypotheses had ring aromatic (RA) while only one hypothesis was made of hydrogen bond donor (HBD). Hypo1 consists of two HBA and three HY-AR features and scored the better correlation and cost difference values. The RMSD value indicates the quality of "prediction" for the training set. The RMSD of all ten hypotheses ranged from 1.176 to 1.421 Å while the Hypo1 showed the lowest RMSD value of 1.176 Å. The correlation coefficient for the Hypo1, 0.942, represents a good correlation by linear regression of the geometric fit index. All these results construe that Hypo1 is the best ranking pharmacophore model among other hypotheses ( Figure 4 ). On the basis of the activity, compounds belonging to training and test sets were categorized into activity scales: most active (++++, IC 50 (inhibitory concentration) < 20 nM); moderately active (+++, ≥20 IC 50 < 200 nM); less active (++, ≥200 IC 50 < 2000 nM); inactive (+, IC 50 ≥ 2000 nM). Activities of all compounds were estimated based on the best ranking pharmacophore model, Hypo1. The experimental and estimated activity values for the 20 training set compounds based on Hypo1 are listed in Table 2 . Analysis of the activity prediction of training set compounds revealed that all the most active compounds were predicted in the same scale, whereas only one moderately active compound was estimated as less active and three inactive compounds were estimated as less active compounds among the 20 compounds of training set. The estimated activity values of most and least active compounds of the training set based on Hypo1 were 0.27 and 4800 nM, respectively, which are very close to that of their experimental activity values (0.46 and 5900 nM). This result revealed that the structural characteristics which can explain the difference in their biological activities are present in Hypo1 ( Figure 5a ). The most active compound 1 could map all the features of the best pharmacophore model, Hypo1, with a fit value of 9.04. The carbonyl oxygen atoms attached with the piperazine ring and azetidinone moiety were mapped onto the two HBA features. All three phenyl rings present in this most active compound mapped over three HY-AR features. The least active compound 20 in the training set maps Hypo1 with a fit value of 4.79 missed two HY-AR features as compared to compound 1. Carbonyl group of imidazolidine-dione and the only carboxyl group of this least active compound mapped both the HBA features whereas the phenyl ring attached to the imidazolidine-dione mapped over one of the HY-AR features ( Figure 5b ).