TET1 and cancer: Studies on TET1 in recent years have focused on cancer, with generally low 5hmC expression levels in patients with breast, lung, liver, gastric and pancreatic cancers, suggesting that downregulation of TET1 expression may contribute to cancer and that TET1 plays a tumor suppressive role in epigenetic modifications (Cimmino et al., 2015; Park et al., 2016; Pei et al., 2016; Collignon et al., 2018; Wu et al., 2019; Zhang et al., 2019).