The RET (rearranged during transfection) proto-oncogene encodes a receptor tyrosine kinase for members of the glial cell line-derived neurotrophic factor family of extracellular signaling molecules. The activating germline point mutations in the RET, which are known to induce oncogenic activation of RET tyrosine kinase, are associated with the development of medullary thyroid carcinoma (MTC) and pathogenesis of multiple endocrine neoplasia type 2 (MEN2). The polypurine/polypyrimidine tract in the proximal promoter region of the human RET gene (-51 to -33 relative to transcription start site) is essential for basal transcriptional activity of this gene. This tract consists of a guanine-rich sequence containing five runs of at least three contiguous guanines separated by one or more bases, conforming to a general motif capable of forming an intramolecular G-quadruplex. Here, we show that specific G-quadruplex structures formed in the RET promoter region act to repress the transcription of this gene, and transcription of this gene can be controlled by ligand-mediated G-quadruplex stabilization. In this study, NSC194598, a derivative of indeno[1,2,3-de]quinazoline, was found to be a novel G-quadruplex interactive agent that interfered with transcriptional activation of mutated RET gene in human medullary thyroid carcinoma TT cells. This compound significantly reduced endogenous RET protein levels and increased apoptosis in these cells. Our results provide further support for the idea that G-quadruplex structures may have a critical role in transcriptional regulation of the RET gene in vivo, providing insight into a novel strategy for transcriptional repression of this gene by small molecules.