Similar with other metallo-b-lactamases, the active site of NDM-1 presents two metal ion binding sites: His site and Cys site. Thus, two zinc ions can be detected in our homology model at both the His and Cys sites with a distance of 4.20 Å apart. According to some experimental data, the one located in the His site has a higher occupancy (1.3 fold) than that in the Cys site (Fig. 3A) . The zinc ion in the His site possesses a tetrahedral coordination sphere and is coordinated by His56, His58, and His125. The other zinc ion in the Cys site has a trigonal-pyramidal coordination sphere that involves Ser11, Asp60, and Cys144.