5-HT reuptake transporter (SERT), as the recycling mechanism for 5-HT, may affect the metabolism in IBS individuals because of its polymorphisms, although there are controversies about the potential relationship between 5-HT, the SERT gene, and IBS (Gershon and Tack, 2007; Cremon et al., 2011; Makker et al., 2015). Here, SLC6A4 is linked to irritable bowel syndrome.