However, in most classes of tumors, HDAC2 overexpression is a worsening effect on tumors, as in the melatonin treatment study of lung adenocarcinoma [28], where melatonin downregulated HDAC2 expression and led to acetylation of histone H3 in two cell lines, A549 and PC9, and enhanced melatonin-induced apoptosis after the addition of HDAC2 siRNA to bring it down. Here, HDAC2 is linked to lung adenocarcinoma.