In conclusion, the present study suggests that the use of letrozole as adjuvant treatment to mild stimulation protocols for IVF may be a safe and cost-effective an effective alternative to standard ovarian stimulation protocols for non-obese PCOS patients undergoing metformin pre-treatment, as it provides comparable IVF outcome without requiring high FSH dose, and avoids supraphysiological estradiol levels, which may be detrimental to the developmental and implantation potential of embryos. This evidence concerns the gene BRD2 and polycystic ovary syndrome.