MYCN and Feingold syndrome: While heterozygous Mycn germline deletion in mice does not cause detectable skeletal abnormalities14, conditional deletion of Mycn in developing limbs (Prx1-Cre:Mycnfl/fl, Mycn cKO) resulted in severe brachysyndactyly similar to that observed in patients with Feingold syndrome type 1 and also in mice previously reported15 (Fig. 1a–d).