Naturally occurring or genetically engineered protein or steroid hormone that binds to a receptor to regulate cell growth, proliferation, or differentiation.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 762 (https://doi.org/10.1515/pac-2015-1202)