4However, the unique therapeutic properties of the MAOIs for patients with so-called atypical depressive features (i.e., hypersomnolence, increased appetite, and weight gain) 6 coupled with the lack of progress in developing effective and welltolerated medications with truly unique mechanisms of action have led to continued efforts to develop safer and easier-to-use MAOIs. The holy grail of this search for a better MAOI has been identification of a drug that (1) did not require a special diet, (2) has a tolerability and ease-of-use profile comparable to the selective serotonin reuptake inhibitors (SSRIs), and (3) is as effective as tranylcypromine or phenelzine. Two aspects of the pharmacology of the older MAOIs—lack of selectivity for the 2 subtypes of MAO (i.e., type A and type B) and the irreversibility of inhibitory effect— have been targets for drug development. The subtyping of the 2 types of MAO is based on the preferred substrates of enzymatic activity. The A type of enzyme, which is found in the gut, liver, and brain, metabolizes norepinephrine, serotonin, and tyramine. Thus, it is inhibition of MAO A