POU5F1B and colorectal cancer: Data supporting positive selection discussed in Section 1.2 have a more qualitative aspect, showing that retrotransposons may increase cell fitness if they hit specific functional genes—if they insert into tumor suppressor (e.g., the MCC gene in hepatocellular carcinomas [55]) or serve as promotors or enhancers of oncogenes (e.g., POU5F1B in colorectal cancer [56] or APOC1 in AML [58]).