Regarding genetic factors, it is worth noting that polymorphisms in the promoter region of the serotonin reuptake transporter (SERT) gene (SERT deletion/deletion genotype) are associated with IBS, particularly diarrhea-predominant [99], and the SERT gene polymorphism of the variable number of tandem repeats is associated with migraine [29,100]. This evidence concerns the gene SLC6A4 and irritable bowel syndrome.