Table 1. Do NO, CO and H₂S qualify as gasotransmitters?

For criteria (iv) to (vi), examples are given.

Criterion	NO	CO	H2S
(i) Small gas	✓	✓	✓
(ii) Membrane-permeable	✓	✓	✓
(iii) Endogenously synthesised	✓	✓	✓
(iv) Specific functions; manipulating endogenous levels evokes changes	✓ antibiotic resistance, virulence, quorum sensing, biofilms, etc.	unclear, except in CO metabolism1	✓ antibiotic resistance, oxidative stress resistance
(v) Effects mimicked by exogenous counterparts	✓	Only toxicity can be mimicked by CO or CORMs?	✓
(vi) Specific cellular/molecular targets2	✓	unclear	Only via persulfide chemistry

¹We do not regard use of CO as a metabolic substrate as signal transduction or gasotransmitter functions; endogenous CO is not used in this way.

²We exclude targets such as metal centres that may lead to toxicity.