Most people have two working copies of each of the MMR genes in their cells. One copy is inherited from the mother and one from the father. A change in the gene that causes it to not work properly is called a mutation. When someone inherits an MMR gene mutation from the mother and another mutation in the same MMR gene from the father, they have constitutional mismatch repair deficiency syndrome. In other words, this person has a mutation affecting each of the 2 copies of an MMR gene.