SANFORD, Fla. -

Prosecutors are winding down their case in George Zimmerman's murder trial after presenting evidence and testimony aimed at refuting the former neighborhood watch volunteer's claim he was acting in self-defense when he fatally shot Trayvon Martin.

Prosecutors are expected to rest their case Friday, a day after jurors return from the Fourth of July holiday. It is still to be seen if they will call Martin's parents to the witness stand.

Casey Anthony attorney Cheney Mason said says Zimmerman's lawyers will have to tread carefully with Martin's mother, Sybrina Fulton.

"The defense better not get up. No objection. No cross-examination. Nothing," Mason said.

If Fulton does testify that she recognizes her son's voice screaming in the background of the 911 calls, the defense is expected to call Zimmerman's father to the stand. Robert Zimmerman Sr. identified the screams as George Zimmerman's.

Medical examiner Shiping Bao has yet to testify. Bao completed Martin's autopsy, which indicated Martin died from a single gunshot wound to the chest and the shot was fired at "intermediate range." He does not define what that means.

Judge Debra S. Nelson has kept the jurors' activities quiet, only stating that there's "something different going for lunch today."

Forensics experts testified Wednesday that Martin's DNA was not found on the grip of Zimmerman's gun, and that Zimmerman's DNA was not found under Martin's fingernails.

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Zimmerman has pleaded not guilty to second-degree murder and says he shot the 17-year-old Martin in the chest to protect himself as Martin reached for his gun during a fight.