However, the associations we observed: (a) the ‘long’ allele 5-HTTLPR genotype, with or without rs25531, would reduce circulating 5-HT level, which is counterintuitive to our hypothesis; (b) carriers of the STin2 VNTR 09 allele, which reduces SERT expression, were more likely to experience diarrhoea, which does fit our hypothesis. Here, SLC6A4 is linked to Diarrhea.