Due to the strong developmental pattern of expression of Dio3 (29–33), mice lacking DIO3 function exhibit pronounced fetal and neonatal thyrotoxicosis (34, 35) leading to reduced testis size (32) and decreased methylation in the neonatal testis that in adulthood translates into an aberrant sperm methylome (36). The gene discussed is DIO3; the disease is thyrotoxicosis.