Mutations resulting in reduced or completely abrogated serotonin-transporter (SERT) function in mice have led to the identification of more than 50 different phenotypic changes, ranging from increased anxiety and stress-related behaviours to gut dysfunction, bone weakness and late-onset obesity with metabolic syndrome. These multiple effects, which can be amplified by gene-environment and gene-gene interactions, are primarily attributable to altered intracellular and extracellular serotonin concentrations during development and adulthood. Much of the human data relating to altered expression of the gene that encodes SERT are based on genetic-association findings or correlations and are therefore not as robust as the experimental mouse results. Nevertheless, SERT-function-modifying gene variants in humans apparently produce many phenotypes that are similar to those that manifest themselves in mice.