Mice Prove Gender Isn't Fixed at Birth - Battle of the sexes rages inside each individual:
"If you think your gender is a fixed part of who you are, a pioneering study suggests you should think again. In it, scientists turned the ovary cells of adult female mice into male testicular tissue by switching off a single gene—one that's found in all mammals. The study unravels the long-held assumption that gender is fixed from birth, implying that in fact the adult body is the site of a constant battle between male and female genes.
The findings could explain biological mysteries such as why some women develop male traits after menopause, and 'may eventually remove the need for surgery in gender-reassignment treatment,' one of the researchers tells the Independent."