In addition, an endogenous reduction in serum and tissue angiotensin-converting enzyme (ACE) levels as a result of polymorphism of the human ACE gene has been associated with an enhanced endurance phenotype13 with the presence of a deletion allele (D) shown to correlate with greater quadriceps strength in COPD.14 A polymorphism determining a reduction in bradykinin receptor expression has also been associated with a reduced fat-free mass and quadriceps strength in patients with COPD.15 Here, ACE is linked to chronic obstructive pulmonary disease.