Although reducing excessive estrogen production via aromatase inhibition was thought to be an alternative strategy for improving outcome of treatment after ovarian stimulation [2], previous studies in humans and rodent models [29, 30] indicate a higher incidence of ovarian cyst formation, a delay or complete failure in ovulation due to inappropriate aromatase inhibitor (e.g., letrozole) treatment. Here, CYP19A1 is linked to Ovarian cyst.