The use of microRNA mimics to increase microRNA levels in the developing organs of newborn mice. Livers from mice treated with 100 mg/kg of an in vivo miR-497a-5p LNA mimic at postnatal day (P)1 and P3 were harvested at postnatal day (P)5. Lungs and livers from mice treated with daily 10 mg/kg intraperitoneal injections of an in vivo miR-497a-5p mimic were harvested at postnatal day 14.