The body determines whether certain genes should be expressed or not. Sometimes genes are only turned on once or twice in a lifetime. Which example best illustrates this point?

A milk-producing hormone is needed mainly during a limited life event, such as after childbirth, so the genes involved may be switched on only once or a few times in a lifetime. Insulin is produced in response to eating, and adrenaline can be released whenever someone is frightened or in danger, so those examples involve repeated activation.

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