When a patent holder delays and does not immediately file a complaint alleging patent infringement, the defendant can allege that the defendant has been prejudiced as a result of laches. Plaintiffs Enzo Biochem Inc., Enzo Life Sciences and Yale University, which hold patents on nonradioactive detection of  nucleic acids, sued Applera Corp. and Tropix Inc., alleging patent infringement of the `767 patent. Applera Corp. filed a motion for summary judgment on an affirmative defense of laches. 35 United States Code §286 provides, "Except as otherwise provided by law, no recovery shall be had for any infringement committed more than six years prior to the filing of the complaint or counterclaim, for infringement in the action." To prevail on laches, a defendant must establish: 1.) the plaintiff delayed filing suit an unreasonable length of time after the plaintiff knew, or reasonably should have known, about the plaintiff's claim; and 2.) the delay resulted in material prejudice to the defendant. The parties disputed whether the plaintiffs knew, or should have known, about the alleged infringement as of June 7, 1998, which was six years before the plaintiffs filed the complaint. The court found that the plaintiffs were aware of a 1997 article in Nucleic Acids Research that described the type of sequencing that the defendant used. The defendant proved that the plaintiffs knew about the defendant's commercial products on or before August 1997 and did not file a complaint until June 2004. Allegedly, the plaintiffs did not immediately sue, because they pursued negotiations. The plaintiffs raised a genuine issue about the reasonableness of the delay in filing a complaint. A jury reasonably could find that the plaintiffs' delay was excusable. The court denied the defendant's motion for summary judgment on the basis of laches.