Everyone has two copies of the mismatch repairs genes (MLH1, PMS2, MSH2 and MSH6). One copy is from their mother and one copy is from their father. If one copy of a mismatch repair gene is not working, this is known as having a faulty mismatch repair gene, or having a mismatch repair mutation.
Children who inherit one faulty mismatch repair gene (either from their mother or their father) have Lynch syndrome and have an increased chance of developing cancer in adulthood.
Children who inherit two of the same faulty mismatch repair genes, one from their mother and one from their father, have CMMRD and have an increased chance of developing cancer from childhood.