The gene is monoamine oxidase A, or MAO-A to its friends.    From Wordnik.com.  [Millions of Useless Purchases Explained at Last!] Reference

MAO-I's are used in the treatment of endogenous depression.    From Wordnik.com.  [Monamine Oxidase Inhibitors] Reference

(That last association is why MAO-A has been dubbed "the warrior gene.").    From Wordnik.com.  [Millions of Useless Purchases Explained at Last!] Reference

About half the U.S. population has at least one low-efficiency MAO-A gene.    From Wordnik.com.  [Millions of Useless Purchases Explained at Last!] Reference

The initial effect of MAO-I reflects a relative increase in available norepinephrine.    From Wordnik.com.  [Monamine Oxidase Inhibitors] Reference