SPP1 and polycystic ovary syndrome: The high levels of OPN in anovPCOS patients could be associated with the levels of oestrogen and androgen, characterising this group of patients as OPN expression is reduced in endometrium cells exposed to the AR inhibitor flutamide and endometrial PCOS cells exhibit an increased expression of ERα nuclear receptor co-activators that may enhance the oestrogen activity in these cells [34, 35].