CDK8 and colorectal cancer: CDK8 may act as an oncogene in colorectal cancer where CDK8 is amplified, with copy number gains observed in ~60% of tumors (Firestein et al., 2010; Seo et al., 2010), and shRNA knockdown can reduce the growth of human colorectal cancer xenografts harbouring CDK8 gene amplification (Firestein et al., 2008; Adler et al., 2012; Starr et al., 2009).