Anaplastic thyroid cancer is one of the most lethal forms of cancer, but the precise carcinogenic mechanism has not been identified. TMEM184C, also known as TMEM34, was identified in a cDNA microarray analysis as being down-regulated in anaplastic thyroid cancers compared to normal thyroid tissues. TMEM184C protein expression was also lower in cell lines derived from these types of cancers compared to that of normal thyroid tissues or cell lines based on other types of thyroid cancers. Furthermore, transfection of TMEM34 into KTA2 cells led to the inhibition of cell growth, suggesting that TMEM184C might act as a tumor suppressor in anaplastic thyroid cancers.