Somatic mutations that lead to tumor development in a clonal, malignant cell line. Principle - defective alleles passed through germ line confer inborn cancer susceptibility, familial cancer syndromes. When inactivated through solely somatic mutations, lead to sporadic tumors. Both ideas involve growth inhibitory signals that cause cell to cease proliferation - machinery that leads to braking cell cycle and division is disabled when both copies of critical gene involved in response to negative elements are mutant. Cell growth could take place, inapproriately, in the presence of inhibitory extracellular signals, if genes involved in control are homozygously absent or mutant.
Second idea is similar, but deals with signals originating intracellularly.
Relate to the ideas of cellular allocation of division cycles - internal clocks. Cells surviving senesence, becoming immortalized, often have mutant forms of antioncogenes, especially p53.