Starting from the complexity of neural pathways and their close integration, this chapter focuses on the possible advantages of investigating polymorphisms involved in the regulation of gene expression or in variation of enzymatic activity, using examples related to neurotransmitter pathway modulation. These examples (that is, serotonin transporter-linked polymorphic region, monoamine oxidase A (MAO-A) promoter region, and catechol-O-methyltransferase (COMT)) are also used to support the hypothesis that quantitative variations of expression and functional levels would be better than structural changes at single receptor sites to identify differences in both treatment response and, likely, psychopathology. Variations in MAO-A activity in female patients are discussed in relation to the pharmacogenetics of panic disorder, together with some hypotheses regarding the chromosomal location of the gene. Finally, the expected impact of new approaches (that is, orphan receptor research, nucleic acid chips, and single nucleotide polymorphisms) is discussed in terms of the advantages and pitfalls.