Epimutations are the most common molecular defect seen in BWS, as 50% of patients exhibit loss of methylation (LOM) at the maternal KCNQ1OT1:TSS-DMR and 5–10% show gain of methylation (GOM) at the maternal IGF2/H19 DMR [13]. The gene discussed is H19; the disease is Beckwith-Wiedemann syndrome.