Tumor suppressor gene

In contrast to the cellular oncogenes, for which
a change in one allele will alter normal function,
both alleles of a tumor suppressor gene must
lose their function before a tumor develops. The
first event is usually a mutation by base exchange
or deletion. The second event, affecting
the other allele (allele 2), may also be a mutation,
but the loss of function more often appears
to be from loss of the chromosome after a faulty
cell division (mitotic nondisjunction) or other
mechanisms