Researchers from China showed that female gene knockout mice lacking either central serotonergic neurons or serotonin prefer female over male genital odors when given a choice, and displayed increased female–female mounting when presented either with a choice of a male and a female target or only with a female target. -Proceedings of the National Academy of Sciences
Click here for the summary.
Sorry guys there are no hot photos of sexing mice in the paper. But there are tons of phallic bar graphs to illustrate genital sniffing and mounting behaviour of the gene knockouts.