Detailed Instructions: In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.
Problem:Question: A key reaction in the regulation of gene expression is acetylation / desatethylation in the histones of some of its residues: 
 Options:  <1> Serina <2> Threonine <3> Lysine <4> Tryptophan <5> To the girl.
Solution:
3