TTF1 and poorly differentiated thyroid gland carcinoma: Compared to poorly differentiated thyroid carcinoma, ATC shows negativity for thyroglobulin (Fig. 7d) and TTF1; although, some authors have reported focal and weak positivity for TTF1 in 10–30% of cases; further, the Ki67 proliferation index is generally high (more than 30%) and p53 is overexpressed in more than 50% of cases [140, 141, 147].