Studies show that serotonin (5-hydroxytryptamine, 5HT) may interact with tumor cell proliferation and tumorigenic signaling pathways. Tryptophan hydroxilase-1 (TPH1), is the rate-limiting enzyme for serotonin production. We determined that the reduction of cellular 5HT levels in MCF7 breast cancer cells by the pharmacological inhibition of the TPH1 enzyme, was associated with growth suppression. To study the relationship between serotonin levels and its proliferative activity we conducted a cloning experiment for the TPH1 gene. The TPH1 gene DNA was cloned in 3 fragments that will be assembled in an expression vector for transfection.