So have most overgrowth syndromes that present with confined overgrowth of one or a few regions of the body (e.g., BWSp Hemihyperplasia, PHTS, PS, PIK3CA, PWS, KTS, PKWS and Proteus syndrome) the molecular defects in only a subset of cells (i.e., a mosaic distribution of the mutation). The gene discussed is PIK3CA; the disease is angioosteohypertrophic syndrome.