1. A signal molecule that influences the fate of cells during embryological development. Inducers activate the expression of specific genes in target cells to bring about differentiation of the latter into specialized tissue cells.

2. A substance that relieves repression of a gene and so allows its expression. For example, in the bacterium E. coli when lactose (or certain related molecules) binds to the repressor of the lac operon, the repressor dissociates from the operator region, allowing transcription of the structural genes encoding the enzymes responsible for lactose metabolism. See operon.