The UniChem schema. The UniChem schema consists of four main tables. Structures are stored in the UC_STRUCTURES table, sources in the UC_SOURCES table. The UC_XREF table contains a list of all src_compound_ids to UCI assignments, and fields to indicate whether these assignments are current or obsolete. The UC_RELEASE table tracks information on data releases for all sources. For clarity, not all fields are shown. Primary/foreign key constraints are indicated by solid arrows. PK = Primary Key, FK = Foreign Key.