Testosterone is a hormone, a molecule released in one set of cells that regulates the action of other cells. Testosterone exerts its action on these target cells by first binding with a receptor, called the androgen receptor (AR). The receptor is a protein that resides within the target cell. The testosterone-receptor complex moves to the nucleus of the target cell, where it acts as a transcription factor. Transcription factors bind to DNA to control the rate of gene expression. In the case of testosterone, the genes affected are those that "masculinize" the fetus, triggering the transformation of the Wolffian ducts into the mature male sexual anatomy and causing other, more subtle changes, including in the brain. Thus, the interaction of testosterone with its receptor is the principal means by which the male genotype (presence of a Y chromosome) leads to the development of the male phenotype (presence of a vas deferens, penis, and accessory structures).
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