In this context, the objective of this study is (i) to evaluate the regulation of cell cycle and DSB repair activity (ii) to explore the therapeutic potential of DNA ligase IV selective inhibition after treatment of anaplastic thyroid cancer cells (C643 and Hth74) with conventional DNA damaging agents used for the treatment of thyroid cancer. The gene discussed is LIG4; the disease is thyroid gland undifferentiated (anaplastic) carcinoma.