Co-author Professor Vincent Harley added: "There is a social stigma that
transsexualism is simply a lifestyle choice, however our findings support
a biological basis of how gender identity develops."

In the latest study, researchers looked for potential differences in three
genes known to be involved in sex development - coding for the androgen
receptor, the oestrogen receptor and an enzyme which converts testosterone to estrogen.

Comparison of the DNA from the male to female transsexual participants
with 258 controls showed a significant link with a long version of the
androgen receptor gene and transsexualism.

http://news.bbc.co.uk/2/hi/health/7689007.stm