Four ways to measure selectivity. (a) The 'selectivity score'  is expressed as a fraction, as signified by the pie chart, and calculated by the formula given. (b) The four steps in calculation of the Gini coefficient  are indicated top-left inside the panel. For simplicity, a 3-protein example is used. The graph shows Gini scores from two inhibitor profiles on 100 kinases. The A'-profile is more specific. The area A' is larger than area A, and therefore the coefficient is larger. (c) The partition coefficient  is a ratio of association constants. The numbers 1, 2, 3... refer to kinases in the profiling panel. If n is a kinase number, then Ka, nis defined as 1/Kd,n. (d) The selectivity entropy. The various colors represent different proteins, and the hexagon a compound. Top: a selective compound binds almost exclusively to one protein, resulting in a narrow distribution across protein species. Bottom: a promiscuous compound binds to many different proteins, resulting in a broad distribution across protein species. The distribution can be quantified using Gibb's entropy definition (the formula shown).