“In mixed stains, we can identify the DNA fragments contributed by the victim which leaves the remaining DNA fragments for comparison with the suspect.”

Many posters ask WHY the foreign/unknown/unidentified DNA at the crime scene was termed "degraded/incomplete/fractured/cracked/old/etc" and JonBenét's was not.

We know the autopsy would have given a full/fresh DNA profile for JonBenét and it would have been compared to the DNA fragments she contributed to the mixed stain. The remaining DNA fragments could then be identitied as attributable to an unknown donor.

We don't know IF the DNA was degraded or contaminated (degraded due to environmental issues, perhaps due to urination at death, or mishandling during collection and storage; or contaminated due to errors in collection or errors in handling at the lab).