BackgroundPTEN hamartoma syndrome (PHTS) is an autosomal dominant disorder characterized by pathogenic variants in the tumor suppressor gene phosphatase and tensin homolog (PTEN). It is associated with an increased risk of muco-cutaneous features, hamartomatous tumors, and cancers. Mosaicism has been found in a few cases of patients with de novo PHTS, identified from blood samples. We report a PHTS patient with no variant identified from blood sample. Constitutional PTEN mosaicism was detected through sequencing of DNA from different tumoral and non-tumoral samples.Case presentationOur patient presented clinical Cowden syndrome at 56 years of age, with three major criteria (macrocephaly, Lhermitte Duclos disease, oral papillomatosis), and two minor criteria (structural thyroid lesions, esophageal glycogenic acanthosis). Deep sequencing ofPTENof blood leukocytes did not reveal any pathogenic variants. Exploration of tumoral (colonic ganglioneuroma, esophageal papilloma, diapneusia fibroids) and non-tumoral stomach tissues found the samePTENpathogenic variant (NM_000314.4 c.389G > A; p.(Arg130Gln)), with an allelic frequency of 12 to 59%, confirming genomic mosaicism for Cowden syndrome.ConclusionsThis case report, and review of the literature, suggests that systematic tumor analysis is essential for patients presentingPTENhamartoma syndrome in the absence of any causal variant identified in blood leukocytes, despite deep sequencing. In 65 to 70% of cases of clinical Cowden syndrome, no pathogenic variant in thePTENis observed in blood samples: mosaicism may explain a significant number of these patients. Tumor analysis would improve our knowledge of the frequency of de novo variations in this syndrome. Finally, patients with mosaicism forPTENmay not have a mild phenotype; medical care identical to that of heterozygous carriers should be offered.