Can an expert give an opinion about the guilt or innocence of a defendant?

"A consistent line of authority in California as well as other jurisdictions holds a witness cannot express an opinion concerning the guilt or innocence of the defendant. [Citations.]... [T]he reason for employing this rule is not because guilt is the 'ultimate issue of fact' to be decided by the jury. Opinion testimony often goes to the ultimate issue in the case. [Citation.] Rather, opinions on guilt or innocence are inadmissible because they are of no assistance to the trier of fact. To put it another way, the trier of fact is as competent as the witness to weigh the evidence and draw a conclusion on the issue of guilt." (People v. Torres (1995) 33 Cal.App.4th 37, 46-47.)

"[A]n expert may render opinion testimony on the basis of facts given 'in a hypothetical question that asks the expert to assume their truth.' [Citation.]" (People v. Gardeley (1996) 14 Cal.4th 605, 618 [hypothetical questions based on facts of case permissible and expert can be asked if actions are gang-related activity done for benefit of the gang].) But the expert cannot exceed the permissible scope of expert testimony simply because he or she is responding to a hypothetical question.