Australian researchers have identified a significant link between a gene involved in testosterone action and male-to-female transsexualism.
DNA analysis from 112 male-to-female transsexual volunteers showed they were more likely to have a longer version of the androgen receptor gene.
The genetic difference may cause weaker testosterone signals, the team reported in Biological Psychiatry.
However, other genes are also likely to play a part, they stressed. Increasingly, biological factors are being implicated in gender identity.
One study has shown that certain brain structures in male-to-female transsexual people are more "female like".
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 oestrogen.
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.