SLC6A4 and gastroesophageal reflux disease: The 5-HTTLPR l/l genotype has greater transcriptional activity, which results in a higher 5-HTT expression level in a synaptic cleft, than the 5-HTTLPR s/s genotype.(35) Therefore, the 5-HTTLPR l/s genotype associated with higher SERT expression level and lower 5-HT level at the synaptic cleft might have exacerbated the GERD symptoms, followed by QOL impairment, in our subjects.