One objective of our study was to assess phenotypic variation associated with the XL allele. Given that the L allele is believed to result in more efficient serotonin function than the S allele, are individuals who possess the XL allele especially resilient to depression? We are not aware of any other study that has addressed this question. Our analysis suggests that the XL allele confers similar or perhaps even greater resilience than the L allele. However, the sampling variability of these estimates coupled with our inability to distinguish the effects of L/XL from L/L genotype, or to analyze variants of the S, L and XL alleles, prevents us from reaching a firmer conclusion.
