A court has been told the most advanced DNA technology has failed to find evidence of sexual contact between George Gwaze and his 10-year-old adopted niece.
He is on trial for the sexual assault and murder of Charlene Mazaka in Christchurch in January 2007.
The Crown says the girl died after a forceful sexual attack by the accused, but the defence says she died of toxic shock arising from her HIV.
ESR scientist Susan Vintiner, an expert in DNA profiling, told the High Court in Christchurch on Monday that traces of semen found on the girl's underwear and skirt could have occurred through everyday household laundering.
Under cross-examination from defence lawyer Jonathan Eaton, she said bodily swabs from George Gwaze, 60, and the girl failed to show any foreign DNA caused by sexual contact between the two.
Ms Vintiner says no foreign DNA was detected inside Miss Mazaka or on a swab taken from the accused.
Previous witnesses have told the court that semen traces matching George Gwaze's DNA were found on the girl's underwear.
Earlier on Monday, a medical expert told the court that Charlene Mazaka died of heart failure caused by an overwhelming septic shock.
Simon Nadel, a paediatric intensive care specialist, gave evidence via video link from St Mary's Hospital in London.
Under questioning by Mr Eaton, Dr Nadel said he believed the girl died from septic shock brought on by her HIV infection.
He disputed evidence given by previous medical witnesses that she had fluid in her lungs, which could have been caused by strangulation.
Instead, Dr Nadel said when the heart is overwhelmed by septic shock, blood backs up into the veins and lungs and that is what appeared in X-rays as fluid in the girl's lungs.
The trial is expected to finish on Friday.