Lin28a −/− males are subfertile. (A–F) Male fertility tests at 7–10 weeks old (A–C) and 7–8 months old (D–F). Number of deliveries (A and D) and number of pups born (B and E) per Lin28a +/+, +/− and −/− male during 13-week experimental period. (C and F) Litter size of Lin28a +/+, +/− and −/− males. N=2–3. (G) Number of plugs per 5-month-old Lin28a +/+, +/− and −/− male during 4-week experimental period. N=3–6. (I–K) Plasma testosterone (I), LH (J), and FSH (K) levels in Lin28a +/+, +/− and −/− males. N=4–10. * p<0.05, ** p<0.01. Error bars represent SEM.