Eligibility Criteria:
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