CHEK2 and lung cancer: Our previous study found that the deubiquitinase USP9X could interact with the histone lysine demethylase 4 C (KDM4C) to mediate KDM4C deubiquitination and thus upregulate its protein expression, and aberrantly expressed KDM4C upregulated the transcriptional expression of TGF-β2 by directly reducing the level of histone H3K9me3 in the promoter region of TGF-β2, which in turn activated the Smad/ATM/Chk2 signaling pathway, promoting DNA damage repair and leading to lung cancer radioresistance [19].