Originally Posted by http://www.dna.gov/training/letraining/glossary.htm

Exclusion - A DNA test result indicating that an individual is excluded as the source of the DNA evidence. In a criminal case, "exclusion" does not necessarily equate to "innocence."

Originally Posted by http://www.dps.state.ia.us/DCI/Crime_Lab/DNA/index.shtml

What can be done with mixtures of DNA from two or more people?

This situation is commonly encountered with sexual assault evidence. DNA extraction procedures are used to separate sperm cell DNA from non-sperm cell DNA. In many cases this is successful and a clear profile of the perpetrator can be obtained. Sometimes complete separation cannot be obtained and the perpetrator's sperm cell DNA will be mixed with the victim's DNA. Blood-blood and semen-semen DNA mixtures can also occur. The ability to distinguish individual contributors to a mixed DNA profile will vary from case to case, and is highly dependent upon the relative quantity of DNA contributed to the mixture by each individual.

Originally Posted by http://www.scientific.org/tutorials/articles/riley/riley.html

The (DNA) profile in this example could have come from a single person whose profile was, AB/AB/AC/AB/AB. Alternatively, two people of types AA/AB/AC/AA/BB and BB/AB/AA/BB/AA if mixed, could produce the profile. There are many other possible combinations of people who, when their DNAs are mixed, could produce the profile. In fact, the only individuals excluded are those possessing the HBGG, B allele and the GC, C allele assuming that the typing strip is reliably detecting all the alleles present. Extreme caution should be used when there is a possibility of a DNA mixture. It is arguable whether the system should be relied upon when there is an unresolved mixture.