The results of this pilot study suggest that a mild stimulation protocol with Letrozole may represent an effective alternative to standard IVF stimulation protocols for non-obese PCOS women pre-treated with metformin, as it provides a comparable IVF outcome to the standard GnRH antagonist protocol, with the advantage of reducing the FSH dose, lowering the estradiol levels under stimulation, and reducing the need of embryo cryopreservation to prevent OHSS. The gene discussed is GNRH1; the disease is ovarian hyperstimulation syndrome.