California Code, Penal Code - PEN § 1127b

When, in any criminal trial or proceeding, the opinion of any expert witness is received in evidence, the court shall instruct the jury substantially as follows:

Duly qualified experts may give their opinions on questions in controversy at a trial.  To assist the jury in deciding such questions, the jury may consider the opinion with the reasons stated therefor, if any, by the expert who gives the opinion.  The jury is not bound to accept the opinion of any expert as conclusive, but should give to it the weight to which they shall find it to be entitled.  The jury may, however, disregard any such opinion, if it shall be found by them to be unreasonable.

No further instruction on the subject of opinion evidence need be given.