One explanation for the incomplete penetrance seen in persons who have inherited cancer susceptibility genes may lie in the effects of modifier genes. Modifier genes are simply genes that indirectly influence the expression of other genes. One can think of the modifier gene as a breaker switch, which controls all of the power. If the breaker switch is off, it doesn't matter whether the susceptibility gene is ready to turn on or not. There is no power, so it won't work. If both copies of modifier genes lead to a breaker switch being thrown, then the gene for cancer susceptibility will be silenced. On the other hand, if the modifier genes power up to let the cancer susceptibility gene be expressed, then there still must be a functioning copy of the gene available.
Here the switch has been thrown OFF by modifier genes and the cancer susceptibility gene is silenced.