Procter & Gamble Company (P&G) (plaintiff) held a patent for the compound risedronate, the active ingredient in P&G’s osteoporosis drug Actonel. Risedronate is a chemical compound known as a bisphosphonate, which is useful in inhibiting the breakdown of bone tissue. After conducting unsuccessful experiments with hundreds of bisphosphonate compounds, including 2-pyr EHDP and 4-pyr EHDP, P&G finally identified risedronate, or 3-pyr EHDP, as a potentially useful compound exhibiting unexpectedly effective attributes. P&G sued Teva Pharmaceuticals USA, Inc. (Teva) (defendant) for infringement of the risedronate patent after Teva communicated its plans to sell risedronate as Actonel’s generic equivalent. Teva contended that the patent was invalid as obvious based on a prior P&G patent listing 2-pyr EHDP as a candidate for treatment of osteoporosis, because 2-pyr EHDP and risedronate were structurally similar. At trial, P&G presented expert testimony establishing that, at the time, the properties of bisphosphonates were highly unpredictable and could not be anticipated based on their structure. The district court found in P&G’s favor, concluding that the patent was valid. Teva appealed.