A significant increase in SERT expression and a reduction in 5-H4R expression will weaken the effects of 5-HT on its effector cells, resulting in decreased post-contractional relaxation of esophageal smooth muscle, impaired coordination of contraction and relaxation, and decreased clearance of refluxed substances, subsequently causing esophageal mucosal damage and esophagitis. The gene discussed is SLC6A4; the disease is esophagitis.