TABLE 1.

Types of ligand bias and their meaning by choice of reference ligand

Type of bias	Reference ligand	Meaning (what can be concluded from data)	Disclaimer (meanings/conclusion not supported by data)
Ligand benchmark‐bias	Any ligand, for example, a candidate drug or tool compound. The reference ligand can be arbitrarily chosen, but often has a particular relevance as tool or clinical agent and is therefore selected to benchmark other tested ligands.	Simultaneous comparison across pathways and ligands where the reference ligand can be any ligand of choice.	A biased ligand for which the reference ligand was not selected based on specific signalling pathway qualities has bias only relative to the reference ligand, which in turn can elicit any bias.
Ligand pathway‐bias	Pathway‐balanced ligand	Signalling preferentially via one pathway, as the reference ligand approximates a pathway‐balanced signal.	A pathway‐balanced/unbiased ligand can be physiology‐biased, although it is by definition unbiased in the pathway definition. A balanced ligand in one system may not be ‘balanced’ in another (applies to all types of ligand bias).
Ligand physiology‐bias	Principal endogenous agonist	Signalling differs from the physiological, as the reference ligand represents the endogenous response of the given receptor and system.	An endogenous agonist can be pathway‐biased, although it is by definition unbiased in the physiological definition.

Note: The terms ‘ligand pathway‐bias’ and ‘ligand physiology‐bias’ are recommended when researchers wish to attribute a specific function (in addition to just a difference to the reference ligand employed).