Pompe disease is a genetic condition which causes muscle weakness over time. People with Pompe disease have a faulty gene that makes an enzyme called acid alpha-glucosidase (or GAA). This enzyme breaks down a type of sugar called glycogen. Without this enzyme, there is a build-up of glycogen in the cells of the body. This causes muscle weakness and other symptoms. Pompe disease can happen at any age, but in late-onset Pompe disease, symptoms generally start from 12 months old onwards. The standard treatment for people with Pompe disease is to receive regular infusions of the GAA enzyme. This is known as enzyme replacement therapy. However, people can build up antibodies against the GAA enzyme over time. Gene therapy is used to treat conditions caused by a faulty gene. It works by replacing the faulty gene with a working gene inside the cells of the body. The working gene is delivered into the cells using certain viruses as carriers (vectors). Viruses are often used as carriers as they can easily get inside cells. The genetic material of the original virus is replaced with the working gene, so only the working gene gets inside the cells. A common virus used as a carrier in gene therapy is the adeno-associated virus (or AAV). This is like an adenovirus, which causes the common cold. The original type of AAV does not cause any harm to humans. However, people that have previously been infected with the original type of AAV may have built up antibodies against AAV. These antibodies may stop the AAV carrier with the working gene getting inside the cells. Researchers want to learn more about antibody levels against AAV and the GAA enzyme in people with late-onset Pompe disease. They also want to learn about other substances in the blood that provide more information about late-onset Pompe disease. These are known as biomarkers. In this study, older teenagers and adults with late-onset Pompe disease will take part. They will not have had gene therapy using AAV. There will be 2 groups - those who have never had enzyme replacement therapy, and those who have had enzyme replacement therapy for 6 months or more. No study treatment will be given during the study, but blood and urine samples will be taken for testing. The main aims of the study are to check antibody levels against AAV8 (a type of AAV) in people with late-onset Pompe disease who had not received any treatment using AAV, to check antibody levels against the GAA enzyme in people previously treated with GAA as part of enzyme replacement therapy, to check levels of biomarkers for Pompe disease, and to check for medical problems. In the study, people will visit the study clinic several times. Some visits may be in the person's home. The first visit is to check if they can take part. Those who can take part will have a medical examination, and have their vital signs checked. Vital signs include blood pressure, heart rate, breathing rate and temperature. Blood samples will be taken to check antibody levels against the GAA enzyme and against AAV8. Blood and urine samples will also be taken to check for biomarkers for Pompe disease. Blood and urine samples will be taken about every 4 months for up to 2 years.