ERCC6 and Cowden syndrome 1: We first isolated human primary fibroblasts from a Chinese CS patient and verified the presence of two nonsense mutations, c.643G>T (p.E215X) in exon 4 and c.3776C>A (p.S1259X) in exon 18, located at different alleles of the ERCC6 gene by genomic DNA sequencing analysis (Fig. 1B).