An effector is a molecule (originally referring to small molecules but now encompassing any regulatory molecule, including proteins) that binds to a protein and thereby alters the activity of that protein. A modulator molecule binds to a regulatory site during allosteric modulation and allosterically modulates the shape of the protein.
An effector can also be a protein that is secreted from a pathogen, which alters the host organism to enable infection, e.g. by suppressing the host's immune system capabilities.
Allosteric effectors can bind to regulatory proteins involved in RNA transcription in order to change its activity.[1] In this way activator proteins become active to bind to the DNA to promote RNA Polymerase and repressor proteins become inactive and RNA polymerase can bind to the DNA.