Analyze each of the following reports, explaining the way in which the method of agreement and the method of difference have been jointly applied, and identifying the special force, if any, of their combination.

A deadly heart ailment affecting about 1 million African American men—familial amyloid cardiomyopathy—and another that afflicts older men of all ethnicities, are known to be caused by an abnormally folded protein that builds up in the organism. Transthyretin protein, made in the liver, has four subunits. A mutation in the gene that makes two of those subunits results in the instability of the protein, its misfolding, and eventually in death. That this is indeed the cause of the ailments was shown by the fact that a liver transplant, providing a healthy version of the critical gene, can result in cure—but often that correction comes too late to stymie the misfolding that did the damage.

A strange twist of nature, reported in Science in January 2003 by Dr. Jeffrey Kelly, of the Scripps Research Institute in San Diego, provided the clues to a therapy that can thwart the misfolding process. Because diseases of this kind are quite common in Portugal, families there are screened to see who has the mutated gene and is therefore at risk. One very large family was identifi ed whose members had the mutated gene and yet never did contract the disease. It turned out that in this family, a second gene that made the other two subunits of the protein had undergone its own mutation, suppressing or reversing the disease process. Members of that family carried a cure to an inherited disease in their own genes. 

Dr. Kelly found that as a result of this further mutation the disease was prevented by the erection of a kind of barrier between the normal and the abnormal protein states. Then, by screening libraries of small molecules, he located several that, already approved by the Food and Drug Administration for other purposes, could mimic the effect of the second mutation, successfuly reversing the misfolded protein in animals.