The activity of a compound in this representation is defined by two counts. Firstly, the number of target bit positions matched sets the potential level of activity -the more positions matched, the greater the activity. Secondly, the target defines a few "kill" bits -matching one or more of these reduces the activity of that compound to zero, regardless of how many target bit positions are matched. The reasoning behind the "kill" bit is to mimic the effect of activity cliffs 9 that are frequently observed in real biological data . In the example below (figure ) the first four bit-positions (blue) are recognized by the assay, conferring activity as the sum of the set blue positions, in this case 1. The last bit-position (red) is a "kill" bit -if it is set the overall activity of the compound is set to zero regardless of the activity sum of the blue positions. The bit-positions in-between (black) can vary without affecting the assay result.