TTF1 and lung adenocarcinoma: Ki67 is widely used as a proliferation marker in pathological assessments, and NK2 homeobox 1, also known as thyroid factor‐1 (TTF‐1), was demonstrated to be frequently suppressed in high‐grade lung adenocarcinoma.[33, 34] The immunohistochemistry results demonstrated that the expression of Ki67 was decreased in tumors derived from A549‐SHMT2‐S90A cells; however, TTF1 expression was higher in tumor‐derived A549‐SHMT2‐S90A cells than in A549‐WT cells (Figure 8G).