The Unique Ingredient Identifier (UNII) is an alphanumeric identifier linked to a substance's molecular structure or descriptive information and is generated by the Global Substance Registration System (GSRS) of the Food and Drug Administration (FDA).
It classifies substances as chemical, protein, nucleic acid, polymer, structurally diverse, or mixture[1][2] according to the standards outlined by the International Organization for Standardization in ISO 11238[3] and ISO DTS 19844.
The GSRS uses molecular structure, protein and nucleic sequences and descriptive information to generate the UNII.
Therefore, two different protein substances can share the same UNII and yet have no biosimilarity or therapeutic equivalence.
[2] The GSRS is a freely distributable software system provided through a collaboration between the FDA, the National Center for Advancing Translational Sciences (NCATS) and the European Medicines Agency (EMA).