Table 2.

Definitions and relevance to uncertainty of the properties relating to structural alerts.

Criteria	Definition and Relevance to Uncertainty
Purpose	The purpose, or potential use, of the structure alert with regard to regulatory assessment, product development etc. and will usually be stated by the user. For low uncertainty the stated use should be clear and unambiguous e.g., for hazard identification relating to toxicity prediction or to facilitate grouping and read-across. The characteristics of the alert should be appropriate for use.
Structural Description	The functional group, or other chemical substructure, that is defined as the structural alert is unambiguously described including any modulating factors and the local molecular environment e.g., substitution patterns on a ring, branching or unsaturation on an alkyl chain etc. Clear and unambiguous definition will enable transparency and documentation.
Property Domain	The domain of the alert defined in terms of relevant physico-chemical properties (e.g., solubility, volatility), molecular descriptors (e.g. 2D, 3D properties such as dimensions), molecular properties (e.g. toxicokinetics (e.g. clearance) and any other relevant property. It is assumed that the domain of the alert will be defined on the training set, if available.
Toxicity or Relationship to Adversity	The definition of the toxicological effect that is elicited, or the adverse effect that may be related to a MIE or KE in an AOP that is associated with the structural alert. This will provide clear indication of the use of the structural alert.
Species Specificity	The structural alert is associated with effects to a particular species, taxa or group of organisms and, if required, life stage.
Metabolic Domain	Consideration of whether the alert requires, or does not require, metabolic activation.
Mechanistic Interpretation	The structural alert is associated with a recognisable and/or understandable mechanism of toxic action, in addition to, where possible, an AOP.
Mechanistic Causality	The definition of the structural alert in terms of structural chemistry, physico-chemical properties etc., is related to the MIE or KE of the mechanism/AOP in a comprehensible/plausible fashion. If possible, the structural alert should relate to the mechanism of action in terms of the chemistry that underpins the interaction with physiological/biochemical processes. E.g. a structural alert for covalent DNA binding should be related to an organic chemistry reactive mechanism. It is noted that an alert may be mechanistically interpretable, but lack mechanistic causality.
Coverage	The coverage is the relative proportion of hits a structural alert would have within a defined chemical inventory.
Performance	The performance of a structural alert can be defined in terms of its predictivity, or ability to match compounds known to be associated with that effect. Ideally structural alerts will have a good prediction rate for positives, and low false positive prediction rate. However, this is dependent, in part at least, on the purpose of the structural alert i.e., toxicity prediction versus grouping or screening.
Corroborating Evidence	The availability of source toxicological, effect or other data that support, or were used to create, the structural alert, e.g., that it may be directly relevant to a toxicological endpoint, adverse effect, MIE etc.
Supporting Evidence	The availability of additional information that may support a weight of evidence approach e.g. data from omics or in vitro assays, or data from other endpoints or non-standard tests, that support the structural alert and provide evidence for the mechanism of action or related to an AOP, but which may not have been considered in the development of the alert. Direct mechanistic relevance may be difficult for many endpoints.