At petitioner’s trial for murder, the prosecution relied heavily on forensic evidence supporting guilt. Petitioner sought to introduce expert testimony that the evidence had been contaminated and that the police were trying to frame him, and evidence that another man committed the crimes. The trial court excluded the third party guilt evidence citing an evidence rule under which the defendant may not introduce proof of third-party guilt if the prosecution has introduced forensic evidence which strongly supports a guilt verdict. The state Supreme Court affirmed. The United States Supreme Court reversed. A criminal defendant’s constitutional rights are violated by an evidence rule under which the defendant may not introduce evidence of third-party guilt if the prosecution has introduced forensic evidence that supports a guilty verdict. The latitude to establish rules excluding evidence from criminal trials is limited by the constitution which guarantees criminal defendants a meaningful opportunity to present a complete defense. The rule here infringed on this interest and was therefore arbitrary or disproportionate to the purposes it was designed to serve.