Inclusion Criteria:  Subjects who are able and willing to provide written informed consent prior to screening; Age of 18 years or older; Able to measure the outcome of the study in this subject.  Part A:  Subjects who start or are on treatment with olaparib tablets 300mg twice daily, according to the drug label and physician's discretion; Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.  Part B:  Subjects who start on treatment with olaparib tablets 300mg twice daily, according to the drug label and physician's discretion; Expected to be on olaparib treatment for ≥ 3 months; ECOG performance status of 0-3.  Exclusion Criteria:  Concurrent use of other anti-cancer therapies; Concurrent use of potent inducers or inhibitors of the cytochrome p450 enzyme 3A3 (CYP3A4) as assessed with the Dutch drug database "G-Standaard" of the Royal Dutch Pharmacists Association(KNMP); Known contra-indications for treatment with cobicistat in line with the summary of product characteristics; Subjects with renal insufficiency defined as estimated glomerular filtration rate < 50 ml/min.