The loss of a single gene turns a normal mouse into a sexual offender and murderer! Solomon Snyder and his team at the John Hopkins University and Massachusetts General Hospital in the US witnessed this in mice whose gene, responsible for the production of nitric oxide (NO), had been blocked (Nature, Vol 378, No 6555). Dissolved NO functions as a messenger between nerve cells in the brain; nNOS is the nerve cell enzyme that produces this neurotransmitter. This disruption makes male mice extremely violent and sexually overactive.