I

Normal growth-stimulating factors

Normal cell \ division

Dominant-acting mutation

Mutation in either allele

Hyperactive stimulatory factor

'Excessive cell proliferation

Normal stimulatory factor rt Proto-oncoaenes normally produce factors that stimulate cell division.

^ Mutant alleles (oncogenes) tend to be dominant: one copy of the mutant allele is sufficient to induce excessive cell proliferation.

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