Using a TALEN dependent targeting of TET1, Kaestner and colleagues simulated the DNA methylation patterns associated with beta cell hyper-proliferation in Beckwith Wiedemann Syndrome (BWS), where hypomethylation of the CDKN1C locus leads to reduced p57Kip2 and beta-cell hyper-proliferation. This evidence concerns the gene TET1 and Beckwith-Wiedemann syndrome.