Important in a number of ways, testosterone must be present in a fetus in order for the child to develop male genitalia. Later in life, during puberty, testosterone is responsible for the further development of the male sex organs, the deepening of the voice, and the growth of facial hair. The hormone is also associated with protein synthesis stimulation and increased muscle development. It is for this latter effect of testosterone that steroids similar to the hormone have been utilized as performance-enhancing drugs, especially by athletes. However, an increased awareness of the harmful side effects of steroid usage may have helped stem their popularity in recent years. Medical use of testosterone includes treatments for frigidity, impotence, and certain varieties of breast cancer.