In People v. Gardeley (1996) 14 Cal.4th 605, the court discussed several principles relating to the use of gang expert testimony. "Under Evidence Code section 801, expert opinion testimony is admissible only if the subject matter of the testimony is 'sufficiently beyond common experience that the opinion of an expert would assist the trier of fact.' The subject matter of the culture and habits of criminal street gangs . . . meets this criterion." (Id., at p. 617.) "Generally, an expert may render opinion testimony on the basis of facts given 'in a hypothetical question that asks the expert to assume their truth.' Such a hypothetical question must be rooted in facts shown by the evidence." ( Id., at p. 618.)