ATC gene expression profile suggested at least three types of undifferentiated cells as its origin: thyroid stem cells, expressing onfFN mRNA but not Tg, with high differentiation potential, self-renewal and ability to generate anaplastic carcinomas; thyroblasts, expressing both Tg and onfFN mRNA and not forming follicles; prothyrocytes, which are more differentiated than thyroblasts, expressing Tg but not onfFN mRNA, with the ability to form follicles. This evidence concerns the gene TG and undifferentiated carcinoma.