FACT database schema. The database schema reflects the generalized handling of heterogeneous data. At the definition layer the data sources are defined as experimental, annotational or analysis sources. Also the types of data that they use are specified here. These types are linked to the individual sources which are defined in the data source layer. Parameters that the functions handling the sources can take are stored as well. The actual data – experimental as well as annotational – are saved as data set and data features in the data set layer.