Trial of Taselisib in Overgrowth

Inclusion Criteria: * Have given, or their legal representative has given, written informed consent to participate * Aged 16 years to 65 years inclusive * Male or female * Post-zygotic PIK3CA mutation * Clinically stable in the opinion of the investigator * Participant Pregnancy and contraception: o Female participants of child bearing potential must use an effective method of contraception during treatment and for at least 3 months after the final dose of taselisib. Acceptable methods are: * True abstinence (this must be the participant's usual and preferred lifestyle, not just for the duration of the trial) * Oral contraceptive (either combined or progestogen alone) * Contraceptive implant, injections or patches * Vaginal ring * Intrauterine device (IUD, coil or intrauterine system) * Condom and cap * Diaphragm plus spermicide * A female participant of child bearing potential is defined as a sexually mature woman not surgically sterilized or not post-menopausal for at least 12 consecutive months if aged 55 years or older. * Men must use one of the following, reliable forms to contraception for the entire duration of treatment and for 3 months after the final dose of taselisib: * Condom plus spermicide even if female partner is using another method of contraception (Men should also use a condom to protect male partners, or female partners who are pregnant or breast feeding, from exposure to the Trial medicine in semen). * True abstinence (this must be the participant's usual and preferred lifestyle, not just for the duration of the Trial) Exclusion Criteria: * Pregnant or breastfeeding * HIV infection * Hypersensitivity to taselisib or any of its excipients * Any current medical disorder or medication likely to impair ability to follow the trial protocol safely and effectively * Are concurrently taking an mTOR inhibitor or any other small molecule inhibitor of the PI3K-AKT signalling pathway * Unable or unwilling to give informed consent * Sirolimus or taselisib treatment in 12 weeks prior to screening * Treatment with a strong inducer or inhibitor of CYP3A4 without the possibility to stop this medication within the week prior to the screening. This includes: * Macrolide Antibiotics: clarithromycin, telithromycin, erythromycin, troleandomycin * Gastrointestinal prokinetic agents: metoclopramide. * Antifungals: itraconazole, ketoconazole, fluconazole, voriconazole, clotrimazole * Calcium channel blockers: verapamil, diltiazem, nicardipine * Grapefruit containing foods/drinks * Anticonvulsants: carbamazepine, phenobarbital, phenytoin * Antibiotics: rifampicin, rifabutin, rifapentine * Herbal preparations: St. John's wort (Hypericum perforatum). Other drugs: bromocriptine, cimetidine, danazol, cyclosporine, lansoprazole, calcium containing antacids. * Inability to attend trial visits * If less than 3 months post- major surgery at screening * Any past medical history of inflammatory bowel disease or chronic diarrhoea of unknown aetiology * History of type 1 or type 2 diabetes mellitus requiring insulin, GLP-1 analogues or oral hypoglycaemic agents. * History of inflammatory bowel disease, ischemic colitis, or colitis of unknown origin. * Fasting blood glucose \> 6.9 mmol/l * HbA1C \> 6% * Long QT, congenital or acquired * Active pneumonitis * Patients who require daily supplemental oxygen * Inadequate renal function defined as creatinine clearance or radioisotope GFR \< 60ml/min/1.73 m² * Inadequate liver function defined as: * Total bilirubin \> 2.0 x ULN or conjugated bilirubin \> 2.0 x ULN for age, and * SGPT (ALT) or SGOT (AST) ≥ 1.5 x ULN for age, and * Serum albumin \< 30 g/L * Inadequate fasting LDL cholesterol \> 4.2 mmol/l * Deprived of freedom by an administrative or court order, or benefiting from a system of legal protection (tutorship, curatorship or safeguard of justice). * Not covered by health insurance