Mutations in the gene encoding glypican (GPC) 3 appear to be responsible for most cases of Simpson–Golabi–Behmel syndrome type 1. Duplication of the GPC4 gene has also been associated to this syndrome; however, no duplications involving GPC3 have been related. We describe a family that harbors a novel exon 2–4 duplication event leading to a truncating germline mutation of the GPC3 gene that, to our knowledge, has not been previously reported. GPC3 transcripts that carry this duplication bear non-functional proteins making its pathogenic role highly probable. The absence of a functional GPC3 may alter the normal differentiation of embryonal mesodermal tissues predisposing to the development of embryonal tumors, as the index case studied who developed a hepatoblastoma at age 9 months. © 2013 Wiley Periodicals, Inc.