Only a small number of the approximately 35,000 genes in the human
genome have been associated with cancer. (See the Genomics unit.)
Alterations in the same gene often are associated with different forms
of cancer. These malfunctioning genes can be broadly classified into
three groups. The first group, called proto-oncogenes, produces
protein products that normally enhance cell division or inhibit normal
cell death. The mutated forms of these genes are called oncogenes.
The second group, called tumor suppressors, makes proteins that
normally prevent cell division or cause cell death. The third group
contains DNA repair genes, which help prevent mutations that lead
to cancer.