Testosterone and other androgens exert their effects on muscle fibers through binding to androgen receptors localized in the sarcoplasm. Androgen receptors are proteins that undergo a conformational change (change shape) when they bind to androgens, allowing them to enter the nucleus of the muscle fiber. Inside the nucleus, the activated androgen receptor binds to specific DNA sequences known as androgen response elements located in the promoter regions of genes involved in muscle growth and protein synthesis. This binding event activates the transcription of RNA molecules from these genes. These newly synthesized RNA molecules are then exported from the nucleus to the sarcoplasm, where they are used as “blueprints” by ribosomes to construct proteins. The increase in protein synthesis mediated by androgens occurs through both increased mRNA transcription and increased translational efficiency. So if an individual has more androgen receptors, the additional proteins synthesized in response to their activation ultimately lead to more muscle growth.