A transcriptional activator is a protein (transcription factor) that increases transcription of a gene or set of genes. Activators are considered to have positive control over gene expression, as they function to promote gene transcription and, in some cases, are required for the transcription of genes to occur. Most activators are DNA-binding proteins that bind to enhancers or promoter-proximal elements. Cyclic adenosine monophosphate (cAMP) is produced during glucose starvation; this molecule acts as an allosteric effector that binds to CAP and causes a conformational change that allows CAP to bind to a DNA site located adjacent to the lac promoter.