Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to 1q21.1 microdeletion ?
    ### Response:
    Most people with a 1q21.1 microdeletion are missing a sequence of about 1.35 million DNA building blocks (base pairs), also written as 1.35 megabases (Mb), in the q21.1 region of chromosome 1. However, the exact size of the deleted region varies. This deletion affects one of the two copies of chromosome 1 in each cell.
    