Catatan

Cancer results from the stepwise accumulation of genetic
alterations within a cell. These alterations lead to abnormal

proliferation and clonal expansion, and ultimately to inva-
sion of surrounding tissues and metastasis to distant sites.

Genetic abnormalities providing a selective advantage are
maintained and ultimately become dominant within the
population. The accumulation of genetic abnormalities,
which in most cases occurs over a period of years, underlies

both the process of tumorigenesis (the transition from nor-
mal cells to invasive cancer) and tumor progression (the

transition to a metastatic and often treatment-resistant can-
cer). Histologic correlates of the process of tumorigenesis

are recognized in a subset of cancers (e.g., colon and blad-
der cancers), whereas for others such cancer-precursor

lesions have not been identified. Dozens of genes involved
in tumorigenesis and tumor progression have now been
identified. The products of these genes regulate key cellular
processes including cell proliferation and survival, cellular

motility and differentiation, and the establishment of cellu-
lar immortality.