Cooperativity of ligand binding as observed in the equilibrium model of ER binding with hormone. (A) Theoretical dependence of hormone binding on the total concentration of the hormone, calculated for 1; 5; 10 and 20 nM of ER. Hb – fraction of the receptor binding sites occupied by hormone. (B) Dependence of the apparent Hill coefficient (nH) on ER concentration. To estimate the values of nH, the theoretical curves shown in Figure 3A were presented in traditional Hill coordinates (bound vs unbound ligand) and then fitted with the Hill equation.