In animals, when feminization occurs in a male, or at an inappropriate developmental age, it is often due to a genetic or acquired disorder of the endocrine system. In humans, one of the more common manifestations of abnormal feminization is gynecomastia, the inappropriate development of breasts which may result from elevated levels of feminizing hormones such as estrogens. Deficiency or blockage of virilizing hormones (androgens) can also contribute to feminization. Interestingly, in some cases, high levels of androgens may produce both virilizing effects (increased body hair, deepened voice, increased muscle mass, etc.) and feminizing effects (gynecomastia) since androgens can be converted to estrogens by aromatase in the peripheral tissues.