Isn't the answer obvious? Neurophilosophy writes:
Researchers create 'lesbian' mice by deleting a single gene
DELETION of a single gene switches the sexual orientation of female mice, causing them to engage in sexual behaviour that is typical of males. Korean researchers found that deleting the appropriately named FucM gene, which encodes an enzyme called fucose mutarotase, causes masculinization of the mouse brain, so that female mice lacking the gene avoid the advances of males and try to mate with other females instead. The findings probably have little relevance to human sexual orientation, however.