We report on somatic mosaicism of a TGFBR2 missense mutation, c.1336G > A (D446N). The affected son with the heterozygous mutation was previously reported [Sakai et al. (2006); Am J Med Genet A 140A:1719–1725]. Further evaluation indicates his clinical condition is Loeys–Dietz syndrome. Parental blood samples were studied to confirm whether the propositus' mutation was a de novo change, and suggested a trace of the mutation in the father. DNAs extracted from blood leukocytes, buccal cells, hair root cells, and nails in the father indicated 52%, 25%, 0%, and 35% of cells harbored the mutation, respectively. This is the first detailed report of somatic mosaicism of a TGFBR2 mutation. © 2008 Wiley-Liss, Inc.