A total of 1,004 DNA profiles taken from crime scenes and loaded to the National DNA Database (NDNAD) between 1 May 2008 and 30 April 2009 matched against more than one subject profile. This equates to an average of 83.7 multiple matches per month over this 12 month period.
The SGM Plus profiling system looks at 10 areas within the DNA molecule and derives from these a profile normally consisting of 20 numbers plus a sex marker indicating whether the person is male or female. With such a profile the likelihood of a random match to a person unconnected to the crime is a minimum of one in a billion.
Biological material found at a crime scene will degrade due to the effects of the environment, ultraviolet light, or bacteria, among other factors. There may be very little DNA in very small stains of biological material. In either case it may not be possible to produce a full DNA profile. In some cases, a partial profile may be produced by the analysis of the DNA and in some cases no DNA profile at all would be obtained. A partial profile consists of fewer than the 20 numbers of a full SGM+ DNA profile. A crime scene profile of eight numbers plus the sex marker would be sufficient to load to the DNA database.
Evidence from such a partial profile may still be crucial, not least because of its ability to conclusively eliminate people from an investigation. A match of a person to such a partial profile carries significantly less weight than a match to a full profile and it is quite possible that two or more people might have the same partial DNA profile. The evidence presented to a court will reflect this. The Crown Prosecution Service have made clear that in any case involving a DNA profile there must be appropriate supporting evidence before a prosecution is initiated.