Reversal was required where the trial court erroneously admitted the testimony of a gang expert to show that graffiti in the defendant’s room was gang-related graffiti. The defendant was not charged with any gang-related offense, but a firearm that was the subject of the charged offense was found in a bedroom that contained graffiti. The defendant did not dispute that the gun belonged to him, and the prosecutor offered no logical connection between the graffiti and any disputed fact in the case. Thus, the court erred in admitting the testimony, and on the facts of the case the error was prejudicial.