"OSSE is tasked by the Federal Ministry of Health to provide patient organizations, physicians, scientists and other parties with open-source software for the creation of patient registries. As a result, the national registry landscape is improved to comply with European principles regarding minimum data set, data quality, etc. (summarized in the EUCERD Recommendation on RD registries). Also, the necessary interoperability is achieved to allow federation of those registries on a national and international level (e.g. distributed searches designed to comply with data protection requirements and preserve data sovereignty). (...) The integration of a metadata repository (MDR) for rare diseases in the OSSE architecture provides semantic interoperability and data quality. All harmonized data sets for rare diseases will be available through the MDR. Further items should be added while building national or regional disease-specific registries and will be available for all OSSE registries. We intend to implement procedures and algorithms relating to quality assurance to avoid uncontrolled growth of MDR content, including duplicates and imprecisely defined items. This implies both intelligent search algorithms to support the retrieval of appropriate existing items and a peer-review approach (e. g. realized by user groups) in order to improve the quality of metadata items. The metadata repository component also allows for the retrieval of metadata items from other MDRs. Apart from self-defined value sets, the MDR will also provide access to standardized codes such as ICD-10 or Orphacode."