The breasts are a pair of mostly fatty tissues which sit on the chest above the pectoral muscles and contain the glands responsible for lactation which exit at the nipples.  Both sexes have breasts but in women they are generally larger, the result of the deposit of more fatty tissues during puberty and are thus considered a secondary sexual characteristic. This is mainly for larger milk content, when nursing infants not weaned. The ratio of estrogen to testosterone in the body is the primary determinant of breast size as they develop.  As such, males with hormone disorders, generally from alcohol damage of the liver or merely old age can develop larger breasts. 

Breast cancer is one of the most common forms of cancer in women and, as such, self-testing and regular mammograms are recommended even in women who have no family history of the disease.  However, although rare, the disease does strike men as well. 

Breast at Wikipedia

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