CDKN1A and esophageal cancer: However, other studies reported that CDKN1A/p21 expression promoted breast cancer and mediated drug resistance (Cheng et al., 2010; Hawthorne et al., 2009), and clinical studies have indicated that high p21 expression was correlated with poor prognosis of gastric and esophageal cancers (Liu et al., 2014; Taghavi et al., 2010).