ID1 and colorectal cancer: Although the Id proteins do not strictly meet the classical definition of oncogenes, as no tumor-associated mutations in the Id genes have been observed, with the exception of Id2 in the colorectal cancer cell line HRT-18 [82] and of Id3 in Burkitt lymphoma [199], the fact that Id-protein up-regulation not only is mediated by oncogenes like Myc, Ras, and (EWS)-Ets, but it also negatively affects tumor suppressor pathways (e.