 The primary study objective is to determine the likely size of sirolimus treatment effect. The patient population will include male and female subjects, aged greater than or equal to 3 years and less than or equla to 65 years of age with segmental overgrowth identified to have clinical and molecular findings of somatic PIK3CA gene mutation. The planned study size will be ten patients seen at the NIHCC. An additional 20 patients will be contributed by two other centers, who will be responsible for the conduct of the proposed research at their site, but the study procedures and dosing schedule will be identical to enable pooling of results for statistical analyses. The study design will be a nonrandomized, open label, phase II pilot study of sirolimus treatment. As patients have highly variable clinical presentations, and there are no established evidence-based methodologies for measuring serial changes in growth, the aim of this pilot study is to establish the optimal methodology for evaluating changes in growth to inform the design of a future randomized controlled trial, in addition to determining treatment effect size, and evaluating safety and toxicity of low dose sirolimus. Overall desired outcome will be reduced size of affected body part, and measures will include: reduction in affected tissue fibroadipose or bone size by clinical exam measurement and by radiological studies MRI area measurements andor DXA study measurements of fat. 