A chemical in the brain controls sexual preference in mice, according to scientists in China.
Male mice bred without serotonin lose their preference for females, a report in Nature says.
The researchers say it is the first time that a neurotransmitter has been shown to play a role in sexual preference in mammals.
Experts have warned about the dangers of drawing conclusions about human sexuality.
The research team first bred male mice whose brains were not receptive to serotonin.
A series of experiments demonstrated that these mice had lost the preference for females shown by unmodified males.
When presented with a choice of partners, they showed no overall preference for either males or females.
BBC News - Sexual preference chemical found in mice
So now we know. It's serotonin. You can change your sexual preference if you wish.