Cells may change sex if programming isn't up-to-date.

Some sex cells change their sex after birth if they aren't reminded how to behave.  In mammals, if ovaries lose the gene Fox12 or if testes lose the gene Dmrt1, some of the dedicated cells change to the other sex.  Reported in Science Daily News.

"This work shows that sex determination in mammals can be surprisingly prone to change, and must be actively maintained throughout an organism's lifetime," said Dr. Susan Haynes.