Susceptibility to depression and a poor response to antidepressants are associated with the presence of a certain allele (S) encoding the serotonin transporter. Bartolomucci et al. show that mice heterozygous for this allele experience more dramatic behavioral changes than normal animals when faced with stress. Serotonin turnover is decreased in these mice, suggesting that a change in serotonin metabolism may underlie a predisposition to depression in individuals with the S form of the serotonin receptor.

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