Androgens mediate a number of diverse responses through the androgen receptor, a 110 kD ligand-activated nuclear receptor. Androgen receptor expression, which is found in a variety of tissues, changes throughout development, aging, and malignant transformation. The androgen receptor can be activated by two ligands, testosterone and dihydrotestosterone, which bind to the androgen receptor with different affinities. This difference in binding affinity results in different levels of activation of the androgen receptor by the two ligands. The androgen receptor acts as a transcriptional modifier of a variety of genes by binding to an androgen response element. The ability to confer androgen specific actions by the androgen response element may depend on other cell-specific transcription factors and cis-acting DNA elements in close proximity to it. Testosterone and dihydrotestosterone appear to act upon an identical nuclear receptor. However, in certain instances, they mediate different physiologic responses. For example, dihydrotestosterone, but not testosterone, is capable of mediating full sexual development of the male external genitalia. In some cases, the androgen receptor may induce opposite physiologic responses in similar tissue types depending on their location. For example, in male pattern baldness, activated androgen receptors may suppress the growth of distinct hair follicle populations through initiating stromal-epithelial actions, whereas other hair follicles continue to proliferate. In other cases, altered androgen receptor activity due to its mutation or altered expression may lead to pathology such as recurrence of prostate cancer due to development of androgen independence allowing tumor cell proliferation under androgen deprivation.