DDB2 and breast cancer: In addition, the level of ubiquitylated DDB2 was decreased in cells transfected with circPFKFB4 overexpression plasmid but increased in cells treated with circPFKFB4 siRNA, suggesting that circPFKFB4 prevented the ubiquitin-proteasomal degradation of DDB2 in hypoxic BC cells (Fig. 6F and G).