This study will examine whether a highly specific and powerful 5-hydroxytryptamine 2A (5-HT2A) antagonist, M100907, combined with escitalopram, is responsible for an antidepressant effect. Major affective disorders are common and can be chronic and life threatening. Yet as many as 50 to 75 percent of patients get only a partial response to the use of antidepressants. Some do not respond to medications in the category of serotonin reuptake inhibitors (SSRIs)-or they experience side effects that sharply interfere with daily life. This study will determine the extent to which M100907 improves sleep and improves fatigue in people who are treated, and how it reduces cognitive impairment, that is, limitations to awareness, in the depressive syndrome. It will also look at allele frequencies as being covariates in the analysis and to collect data.