AR diseases are those that manifest clinically only when the patient is homozygous for the illness (i.e., both copies of the gene are mutant). AR diseases are characterized by the following inheritance pattern (see Fig. 1-7 B): 1. Affected children's parents may be clinically normal (i.e., carriers). 2. Carriers can pass on the mutated gene to their offspring. 3. If one receives a single copy of the mutant gene, one will also receive it from one's other parent. Thus, affected individuals arise in families where both parents are carriers. The most common form of AR disease is cystic fibrosis (CF). CF is an autosomal recessive disease caused by mutations in the CFTR gene. The CFTR gene produces a protein called CFTR which functions as an ion channel in cells of many organs including the lungs and intestines. People with two copies of the mutated CFTR gene do not make enough CFTR protein to open up their cell membranes properly which leads to bacterial colonization of the pancreas and inflammation of the liver. In addition, people with CF have high levels of sodium in their sweat which indicates that there is no CFTR protein present in the skin. Another type of AR disease is Duchenne muscular dystrophy (DMD). DMD is a severe muscle-wasting disease that usually affects boys before they reach their teenage years.