This study has several parts and involves taking a study drug called alpelisib. Some participants enrolled into Stage 2 will receive a placebo for the first part of the study (24 weeks) and will then receive the study drug. The purpose of this study is to find out if the study drug is safe and effective (can help) people with a confirmed diagnosis of lymphatic malformation (LyM) and a documented mutation in the PIK3CA gene

The purpose of this study is to find out if alpelisib (BYL719; the "study drug"), is safe and effective (can help) people who have PIK3CA-related overgrowth spectrum (PROS) disease.