[Mosaicism as a cause of Cowden syndrome].

Cowden syndrome (CS) is a hereditary cancer predisposition syndrome characterised by macrocephaly, mucocutaneous lesions and increased cancer risk in the breast, thyroid, endometrium, kidneys, and colon. We report in this case report, we present a patient clinically diagnosed with CS in whom genetic analyses yielded normal results. In DNA isolated from a buccal swab rather than a standard blood sample, we identified a known pathogenic PTEN missense variant in mosaic form using whole-genome sequencing. Genetic mosaicism should be considered in patients with suspected monogenic cancer predisposition and negative genetic testing.