SLC6A4 and pulmonary arterial hypertension: The overexpression of 5-HTT seen in the pulmonary vessels of patients with idiopathic PAH is at least partly related to an insertion/deletion polymorphism in the promoter region of the 5-HTT gene with long (L) and short (S) forms: the L allele drives a 2- to 3-fold higher rate of 5-HTT gene transcription than does the S allele.