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The immune system produces antibodies called antigens. When a virus enters the body, the immune system generates antibodies to attack it. In a successful attack, the antibodies target and bind to the antigen, stopping it from causing harm to the body. Antibodies are incredibly diverse. Some scientists estimate that there may be as many unique antibodies as there are stars in the galaxy. Antibodies are made up of amino acids, and scientists commonly identify a particular antibody according to its specific sequence of amino acids-what they call an antibody’s “‘primary structure.’” As the atoms of the amino acids interact with each other, they create folds that result in complex three-dimensional shapes. Scientists refer to an antibody’s intricate topography as its “tertiary structure.” An antibody’s structure does much to dictate its function-its ability to bind to an antigen and, in some instances, to block other molecules in the body from doing the same. “For an antibody to bind to an antigen, the two surfaces have to fit together and contact each other at multiple points.” To bind and block, the antibody must establish a sufficiently broad, strong, and stable bond to the antigen. Different antibodies have different binding and blocking capacities based on the amino acids that compose them and their three-dimensional shapes. Changing even one amino acid in the sequence can alter an antibody’s structure and function. Scientists cannot accurately predict exactly how trading one amino acid for another will affect an antibody’s structure and function. Translating at sequence into a known three-dimensional structure is still within the realm of conjecture and “black art.” Scientists are now able to engineer antibodies to assist in treating diseases. Some of these lab-made antibodies target not foreign agents but the body’s own proteins, receptors, and ligands. Proteins, receptors, and ligands can also be involved in inflammatory disorders, uncontrolled cell growth, or other biological pathways that may be associated with disease. Scientists have focused on the creation of antibodies to treat patients with high LDL cholesterol. LDL cholesterol can contribute to the formation of plaque in the arteries that may lead to cardiovascular disease, heart attacks, and strokes. For many with high LDL cholesterol, drugs called statins offer an effective treatment. For others, statins do not work well or come with unwelcome side effects. In those cases, a relatively new antibody-based treatment known as a PCSK9 inhibitor may be appropriate. PCSK9 is a naturally occurring protein that binds to and degrades LDL receptors. The body produces LDL receptors to perform the beneficial function of extracting LDL cholesterol from the bloodstream. Scientists are now exploring how antibodies might be used to inhibit PCSK9 from binding to and degrading LDL receptors as a way to treat patients with high LDL cholesterol. A number of pharmaceutical companies began to create antibodies that could bind to a particular region of PCSK9 called the “sweet spot.” The sweet spot is a sequence of 15 amino acids out of PCSK9’s 692 total amino acids. By binding to the sweet spot an antibody could prevent PCSK9 from binding to and degrading LDL receptors. P developed a PCSK9-inhibiting drug that it marketed under the name Repatha, and d produced one it labeled Praluent. Each drug employs a distinct antibody with its own unique amino acid sequence. P obtained a patent for the antibody employed in Repatha, and Sanofi received one covering the antibody used in Praluent. Each patent describes the relevant antibody by its amino acid sequence. P obtained two additional patents that relate back to the company’s 2011 patent. This case revolves around claims 19 and 29 of the ’165 patent and claim 7 of the ’741 patent. P did not seek protection for any particular antibody described by amino acid sequence but claimed “the entire genus” of antibodies that (1) “bind to specific amino acid residues on PCSK9,” and (2) “block PCSK9 from binding to [LDL receptors].” P identified the amino acid sequences of 26 antibodies that perform these two functions, and it depicted the three-dimensional structures of two of these 26 antibodies. P detailed two methods to make other antibodies that perform the binding and blocking functions it described. The roadmap method directs scientists to: (1) generate a range of antibodies in the lab; (2) test those antibodies to determine whether any bind to PCSK9; (3) test those antibodies that bind to PCSK9 to determine whether any bind to the sweet spot as described in the claims; and (4) test those antibodies that bind to the sweet spot as described in the claims to determine whether any block PCSK9 from binding to LDL receptors. The “conservative substitution” method requires scientists to: (1) start with an antibody known to perform the described functions; (2) replace select amino acids in the antibody with other amino acids known to have similar properties; and (3) test the resulting antibody to see if it also performs the described functions. P sued D for infringing the ’165 and ’741 patents. D claimed that the patents were invalid as a matter of law because P had not enabled a person skilled in the art to make and use all of the antibodies that perform the two functions Amgen described in its claims. P’s claims cover potentially millions more undisclosed antibodies that perform these same functions. D argued neither of the two methods outlined for generating additional antibodies with the same functions enables a person skilled in the art to do so reliably. D claimed that P detailed a trial-and-error process of discovery. The court held that “no reasonable factfinder could conclude” that P had provided “adequate guidance” to make and use the claimed antibodies “beyond the narrow scope of the working examples” it had identified by their amino acid sequences. The court of appeals affirmed. P appealed.

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