We amplified, using the polymerase chain reaction and calcitonin receptor (CTR) specific primers, RNA extracted from medullary thyroid carcinoma (MTC) and the derived TT cell line. Both secrete large amounts of calcitonin. Electrophoresis of amplification products revealed, in both cases, an ethidium bromide-stained band that hybridized to a CTR probe. Sequencing the band amplified from TT cells revealed an open reading frame identical to the sequence of H-CTR but lacking 16 amino acids in the first intracellular loop. This demonstrates the existence of an mRNA coding for a subtype of H-CTR which is expressed in TT cells and MTC.