After adding 5–10 % of BWS patients positive for loss-of-function CDKN1C mutations and a small (1–2 %) fraction of 11p familial or de novo rearrangements causing duplication of the paternal chromosome, up to 20 % of clinically diagnosed BWS cases remain without a molecular diagnosis [8, 9]. This evidence concerns the gene CDKN1C and Beckwith-Wiedemann syndrome.