SLC6A4 and irritable bowel syndrome: About one-third of IBS patients have a positive family history. Twin studies have shown higher concordance of IBS in monozygotic than dizygotic twins. Mutation and gene polymorphism of serotonin receptors such as Serotonin reuptake receptor (SERT) and sucrose isomaltase (SCN5A) have been reported in IBS [17,18].