An effector molecule is usually a small molecule that selectively binds to a protein and regulates its biological activity. In this manner, effector molecules act as ligands that can increase or decrease enzyme activity, gene expression, or cell signalling. Effector molecules can also directly regulate the activity of some mRNA molecules
In some cases, proteins can be considered to function as effector molecules, especially in cellular signal transduction cascades.
The term effector is used in other fields of biology. For instance, the effector end of a neuron is the terminus where an axon makes contact with the muscle or organ that it stimulates or suppresses.