The court did not err in refusing to instruct the jury regarding witness credibility under CALJIC 2.20 where the evidence offered to discredit the witness was not character evidence but evidence regarding the witness’s alleged mental illness. An expert witness testified that the complaining witness suffered from borderline personality disorder; the testimony described the symptoms of that disorder and suggested that the witness might be capable of fabricating information during stressful times. Over the defendant’s objection, the judge struck the portion of CALJIC 2.20 that permitted the jury to consider the character of the witness for honesty or truthfulness, and the appellate court held that this was not error. The expert’s testimony did not constitute character evidence and was not an appropriate subject for such an instruction.