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1. That which carries a message.
2. Having message-carrying properties.
1. Pertaining to ribonucleic acid (RNA) that carries the coded information for protein synthesis from the DNA to the site of protein synthesis, the RIBOSOMES.
2. A HORMONE or other effector capable of acting at a distance from its site of production. A second messenger is a hormone, produced within a cell and operating on internal structures, when another hormone acts on the outer cell membrane.