Did the Court of Appeals err in considering post-trial DNA evidence when assessing whether the Florida Supreme Court reasonably determined that a witness's testimony was immaterial to the jury's verdict? Specifically, can evidence not presented to the jury be used to determine if a constitutional error influenced the jury's verdict? This inquiry involves the application of the Antiterrorism and Effective Death Penalty Act of 1996 and the standard for federal habeas relief.

• A jailhouse informant's testimony regarding his lack of criminal history was false, and the prosecution knew it was false because it possessed his juvenile records.
• This false testimony violated his due process rights under *Giglio v. United States*.
• The *Giglio* error had a "substantial and injurious effect or influence in determining the jury's verdict" as required for relief under *Brecht v. Abrahamson*.
• The Court of Appeals improperly relied on post-trial DNA evidence to conclude that the error was harmless.

• The Florida Supreme Court reasonably determined that the evidence against the petitioner was "overwhelming" even without the informant's testimony.
• Under 28 U. S. C. § 2254(d)(2), the federal court must defer to the state court's determination unless it was "based on an unreasonable determination of the facts in light of the evidence presented."
• The petitioner failed to exhaust his *Giglio* claim in the state courts.

• The Court vacated the judgment of the Court of Appeals.
• The Court held that the Court of Appeals erred by considering post-trial DNA evidence to assess whether the Florida Supreme Court's determination was reasonable.
• The Court applied the standard from *Brecht v. Abrahamson*, which asks whether a constitutional error had a "substantial and injurious effect or influence in determining the jury's verdict."
• The Court reasoned that because the post-trial DNA evidence was not presented to the jury—and did not exist at the time of the trial—it could not have influenced the jury's verdict.
• Because the evidence could not have influenced the verdict, the Court concluded it "sheds no light on whether (or to what extent)" the informant's testimony influenced that verdict.
• The Court noted that under 28 U. S. C. § 2254(d)(2), the Court of Appeals was required to assess the reasonableness of the state court's determination "in light of the evidence presented at trial."
• The Court expressed no view on whether the Florida Supreme Court's determination was reasonable based solely on trial evidence, leaving that for the Court of Appeals to assess.
• The Court declined to rule on the State's argument regarding the failure to exhaust claims, citing *Cutter v. Wilkinson* to state that the Supreme Court is a "court of review, not of first view."