sentences:  Because these sequences vary in the population, the actual number of copies of short sequence repeats does not matter, just that the number the patient does have is consistent from tissue to tissue and over time.This phenomenon occurs because these sequences are prone to mistakes by the DNA replication complex, which then need to be fixed by the mismatch repair genes.If these are not working, over time either duplications or deletions of these sequences will occur, leading to different numbers of repeats in the same patient.