As the name suggests, these are rare diseases whose clinical pictures are often poorly understood. This is why, when symptoms appear that may indicate the presence of a rare disease, the appropriate diagnostic tests should be carried out in order to begin optimal care and therapy. Advances in various scientific fields, especially in genetics, make it possible to identify the cause of an increasing number of these diseases and to make the correct diagnosis. Genetic studies to diagnose rare diseases include: Single gene analysis: In case of suspicion of a specific rare disease, either by symptomatology, traits and/or family tree, analysis of specific mutations in certain genes known to be associated with the disease in question can be performed. Exome or genome: In cases where the symptomatology, traits and family tree cannot be associated with a specific rare disease, it is advisable to perform an exome or genome analysis. These are genetic studies that make it possible to analyse all the genes of an individual (exome) or all the nuclear DNA of an individual (genome). This significantly increases the probability of identifying the mutation(s) that may be the cause of the rare disease.