Final answer:
Initiator caspases activate executioner caspases in response to an apoptotic stimulus.
Step-by-step explanation:
In response to an apoptotic stimulus, initiator caspases activate executioner caspases. Initiator caspases are responsible for initiating the process of apoptosis by cleaving and activating executioner caspases, which eventually leads to cell death. They also play a role in amplifying the apoptotic signal by cleaving and activating other proteins involved in the apoptotic process.