The terms "strong" and "weak" are often used to describe promoters and enhancers, according to their effects on transcription rates and thereby on gene expression. Alteration of promoter strength can have deleterious effects upon a cell, often resulting in disease. For example, some tumor-promoting viruses transform healthy cells by inserting strong promoters in the vicinity of growth-stimulating genes, while translocations in some cancer cells place genes that should be "turned off" in the proximity of strong promoters or enhancers.
Enhancer sequences do what their name suggests: They act to enhance the rate at which genes are transcribed, and their effects can be quite powerful. Enhancers can be thousands of nucleotides away from the promoters with which they interact, but they are brought into proximity by the looping of DNA. This looping is the result of interactions between the proteins bound to the enhancer and those bound to the promoter. The proteins that facilitate this looping are called activators, while those that inhibit it are called repressors.