inducer

noun

1. Biochemistry. a substance that has the capability of activating genes within a cell.
2. Embryology. a part that influences differentiation of another part.

n.

1. One that induces, especially a molecule that is usually a substrate of a specific enzyme pathway, combining with an active repressor produced by a regulator gene to deactivate the repressor.
2. A part or structure in an embryo that influences the differentiation of another part.