Carbon monoxide is a one-carbon compound in which the carbon is joined only to a single oxygen. It is a colourless, odourless, tasteless, toxic gas. It has a role as a neurotoxin, a signalling molecule, a vasodilator agent, a neurotransmitter, a metabolite, a P450 inhibitor, a ligand, a biomarker, a probe, a human metabolite, a mouse metabolite, an EC 1.9.3.1 (cytochrome c oxidase) inhibitor and a mitochondrial respiratory-chain inhibitor. It is a one-carbon compound, a gas molecular entity and a carbon oxide. It is a conjugate base of a carbon monoxide(1+).