Abstract: Estrogen Receptor belongs to the Nuclear receptor family, there are two different currently known subtypes Era and Erb. Levels and proportion of the two subtypes differs in different target cells. We can hypotheses a different pharmacological activity upon ligand binding. The aim of this work is the investigation through molecular dynamics simulations, on both the subtypes a and b of the estrogen receptor, of possible differences among them and for the study of their dynamical behaviour.