Side effects of clozapine, an antipsychotic drug, have been linked to the gene for the serotonin (5-HT) 5-HT2A receptor, HTR2A. Many antidepressants also affect serotonin, and their side effects thus might also be related to individual variation in the HTR2A gene (genotypes). Another gene, CYP2D6, controls activity of an enzyme that is important in the metabolism of many antidepressants, including paroxetine and mirtazapine. Murphy et al. (p. 1830) examined the influence of HTR2A and CYP2D6 genotypes on side effects in elderly depressed patients taking paroxetine and mirtazapine, which activate and block 5-HT2A receptors, respectively. CYP2D6 genotype, determined with a high-throughput microarray technique, was not associated with side effects from either medication, and HTR2A genotype did not influence side effects from mirtazapine. However, patients taking paroxetine who had two C alleles at the HTR2A 102 T/C single nucleotide polymorphism (C/C genotype) had worse side effects and were more likely to stop treatment than were patients with the T/C or T/T genotype.