XUEC The XUECs are Networks of Units of Clinical Expertise [Xarxes d’Unitats d’Expertesa Clínica, in Catalan] in rare diseases which seek to collaborate with each other and share knowledge. They coordinate with healthcare and social care units to provide a more personalised and efficient service. A disease is considered to be ‘rare’ when it affects fewer than 1 in every 2,000 inhabitants, has a high mortality rate and/or is chronically debilitating. The low or very low prevalence of rare diseases requires a particular concentration of resources and knowledge. The expertise, therefore, needs to be concentrated into different reference units. These have been grouped into thematic clusters and areas, taking into account the Rare Disease groups established by EURORDIS. The classification of Unit of Clinical Expertise (UEC) is based on an evaluation of the following criteria: Knowledge and experience Multi-disciplinary team Functional plan of the UEC Transition protocol in paediatric UECs List of XUEC accreditations Adult and childImmune diseases: area of primary immunodeficienciesImmune diseases: area of autoinflammatory diseasesImmune diseases: area of systemic autoimmune diseasesRare diseases with epilepsyRare Endocrine DisordersNon-Oncological Minority Hematological DisordersHereditary metabolic diseasesNeuromuscular diseasesKidney diseaseRespiratory rare diseasesOphthalmic rare diseasesCardiac rare diseasesPediatricGenetically based behavioral cognitive disorders in pediatric age