A paternal uniparental disomy (UPD) of 11p15.5 accounts for approximately 20% of BWS cases, leading to a biallelic expression of IGF2 and reduced expression of both H19 and CDKN1C. Primarily sporadic mutations in CDKN1C are observed in approximately 5% of BWS cases. The gene discussed is CDKN1C; the disease is Beckwith-Wiedemann syndrome.