NKX2-1 and medullary thyroid gland carcinoma: Medullary thyroid carcinoma cells may be spindle-shaped; however, immunohistochemically, they are reactive for keratins, thyroid transcription factor-1 (thyroglobulin-negative), neuron-specific enolase, chromogranin (A, B and C), synaptophysin, opioid peptides and calcitonin.