The study reports the molecular cloning, expression, and chromosomal localization of a human serotonin transporter (hSERT) that is sensitive to antidepressants and cocaine. The hSERT gene was identified using PCR from human placental cDNA and was found to be highly related to the rat brain 5HT transporter. Transfection of the cloned hSERT into HeLa cells resulted in the expression of a high-affinity, Na+ and Cl− dependent 5HT transport activity that was blocked by selective 5HT transport inhibitors and antagonized by cocaine and amphetamine. Sequence analysis revealed a 630-amino acid open reading frame with 92% identity to the rat brain 5HT transporter, including identical predicted topological features and conserved sites for posttranslational modifications. Unlike rodents, where a single mRNA appears to encode the 5HT transporter, multiple hybridizing RNAs were observed in human placenta and lung. Somatic cell hybrid and in situ hybridization studies confirmed the presence of a single gene encoding the human 5HT transporter, localized to chromosome 17q11.1-17q12. The findings suggest that alterations in the hSERT gene may play a role in human neuropsychiatric disorders, placental or vascular physiology, and differential sensitivity to medications and addictive substances.The study reports the molecular cloning, expression, and chromosomal localization of a human serotonin transporter (hSERT) that is sensitive to antidepressants and cocaine. The hSERT gene was identified using PCR from human placental cDNA and was found to be highly related to the rat brain 5HT transporter. Transfection of the cloned hSERT into HeLa cells resulted in the expression of a high-affinity, Na+ and Cl− dependent 5HT transport activity that was blocked by selective 5HT transport inhibitors and antagonized by cocaine and amphetamine. Sequence analysis revealed a 630-amino acid open reading frame with 92% identity to the rat brain 5HT transporter, including identical predicted topological features and conserved sites for posttranslational modifications. Unlike rodents, where a single mRNA appears to encode the 5HT transporter, multiple hybridizing RNAs were observed in human placenta and lung. Somatic cell hybrid and in situ hybridization studies confirmed the presence of a single gene encoding the human 5HT transporter, localized to chromosome 17q11.1-17q12. The findings suggest that alterations in the hSERT gene may play a role in human neuropsychiatric disorders, placental or vascular physiology, and differential sensitivity to medications and addictive substances.