Understanding rare disease reporting can feel overwhelming when scientific, clinical, and regulatory language appears without explanation. This glossary has been created to make that world easier to navigate. It brings together key terms used across research, diagnostics, policy, funding, and patient care so that readers can follow developments with confidence.
Each definition is written in plain English, with enough detail to be accurate without becoming technical jargon. Whether you are a patient, parent, researcher, clinician, advocate, policymaker, or simply someone interested in rare conditions, this resource offers a reliable starting point.
You will find explanations for essential concepts such as biomarkers, orphan designation, natural history studies, prevalence, phenotypes, accelerated approval pathways, post marketing surveillance, and many others that regularly appear in our articles.
New terms are added as the field evolves and as readers request further clarity. The aim is to support informed reading, reduce confusion, and help bridge the gap between specialists and the wider community.
Accelerated Approval
A regulatory route that allows medicines for serious conditions to be approved earlier when evidence suggests they are likely to provide benefit. Further studies are required after approval to confirm effectiveness.
Advocacy Group
An organisation that supports people affected by a condition, providing information, community, and representation in policy or research discussions.
Allele
A version of a gene. People inherit two alleles of most genes, one from each parent.
Biobank
A collection of biological samples such as blood or tissue, stored for research to help identify causes of disease and potential treatments.
Biomarker
A measurable indicator of a biological process. Biomarkers can be used for diagnosis, tracking disease progression, or monitoring responses to treatment.
Burden of Disease
The overall impact of a condition on individuals and society, including health effects, quality of life, and economic cost.
Case Report
A detailed description of an individual patient’s symptoms, diagnosis, and treatment. Case reports are important in rare disease research because small numbers limit large trials.
Clinical Endpoint
A measurable outcome used to judge whether a treatment works, such as improved mobility or reduced seizure frequency.
Clinical Trial
A structured study testing whether a treatment is safe and effective in humans.
Cohort Study
An observational study following a group of people over time to track disease development, risk factors, and outcomes.
Compassionate Use
A programme that allows patients to access experimental treatments outside of clinical trials when no approved options exist.
De Novo Mutation
A genetic change that appears for the first time in an individual rather than being inherited from a parent.
Differential Diagnosis
A list of possible conditions that could explain a patient’s symptoms. This is vital in rare disease where many conditions look similar.
Efficacy
The ability of a treatment to produce the desired effect in a controlled study.
Endpoint Validation
The process of confirming that a chosen outcome measure accurately reflects meaningful change for patients.
Epidemiology
The study of how diseases occur in populations, including prevalence, incidence, risk factors, and patterns over time.
Exome
The small portion of the genome that codes for proteins. Exome sequencing is often used to diagnose rare disorders.
Genetic Carrier
A person who has one altered copy of a gene associated with a recessive condition. Carriers usually have no symptoms but can pass on the change.
Genetic Counselling
Professional support that helps individuals understand genetic testing, results, and associated risks.
Genotype
A person’s genetic makeup for a specific gene.
Heterozygous
Having two different alleles of a gene.
Homozygous
Having two identical alleles of a gene.
Idiopathic
A condition with no identified cause.
Incidence
The number of new cases of a condition diagnosed during a specific period.
Informed Consent
A process ensuring that people understand what participation in a study or treatment involves before agreeing to take part.
In Silico Study
Research carried out using computer modelling rather than laboratory experiments.
In Vitro Study
Experiments performed outside a living organism, for example in a test tube or petri dish.
In Vivo Study
Experiments performed within a living organism.
Investigational Medicine
A treatment that is being tested but is not yet approved.
Longitudinal Study
A study that follows the same individuals over time to observe how a condition develops or responds to treatment.
Metabolic Disorder
A condition caused by problems with the body’s chemical processes, often due to missing or faulty enzymes.
Mitochondrial Disorder
A group of conditions caused by changes in genes that affect how cells produce energy.
Multidisciplinary Team
A group of healthcare professionals with different expertise working together to support a patient.
Natural History Study
A study that tracks how a disease develops without treatment. This is crucial in rare diseases to inform trial design and measure treatment impact.
Next Generation Sequencing
Advanced genetic testing that can read large amounts of DNA quickly and cost effectively.
Orphan Designation
A special regulatory status granted to medicines intended for rare conditions, offering incentives such as fee reductions and market exclusivity.
Pathogenesis
The biological process that leads to the development of a disease.
Patient Registry
A structured database that collects information from people with a specific condition to support research, care improvement, and policy planning.
Peer Reviewed Article
A scientific paper that has been evaluated by independent experts before publication.
Phenotype
The observable features or symptoms of a person’s condition, influenced by both genetics and environment.
Pharmacovigilance
Monitoring the safety of medicines after they are approved and used in wider populations.
Preclinical Study
Research conducted before human trials, usually involving laboratory or animal models.
Prevalence
The total number of people living with a particular condition at a given time.
Progressive Condition
A condition that becomes more severe over time.
Protocol
A formal plan for a study that outlines its design, objectives, procedures, and analysis.
Quality of Life Measurement
Tools that assess how a condition affects daily living, wellbeing, and social participation.
Randomised Controlled Trial
A clinical study in which participants are assigned at random to receive either the treatment being tested or a comparison option.
Regulatory Approval
Official authorisation for a medicine to be marketed after its safety and effectiveness have been reviewed.
Sequencing
The laboratory process of reading the order of DNA building blocks.
Somatic Mutation
A genetic change that occurs after conception and is not inherited.
Spectrum Disorder
A condition that can present with a wide range of symptoms and severities.
Standard of Care
The best treatment approach based on current evidence and clinical practice.
Targeted Therapy
A treatment designed to act on a specific gene, protein, or pathway involved in a disease.
Therapeutic Window
The range between a dose that works and a dose that causes harm.
Ultrarare Disease
A condition affecting very small numbers of people, often defined as fewer than one patient per fifty thousand.
Variant of Uncertain Significance
A genetic change identified in testing where its impact on health is not yet known.
Whole Genome Sequencing
A test that reads nearly all of a person’s DNA to search for disease causing changes.
