Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

Q: Sentence1: Cell death occurs by activation of cell death genes. 
Sentence2: Aging is associated with the activation of certain genes.

A: activation
****
Q: Sentence1: Protein Protein is needed for tissue maintenance and repair. 
Sentence2: protein repairs cells.

A: Protein
****
Q: Sentence1: threatening behavior causes a fight-or-flight response in animals. 
Sentence2: adrenaline is a response to threats.

A:
threatening
****