Semen DNA analysis: The detection and investigation in a sample of semen. Let us assume that a man's semen is alleged to be on a piece of fabric. To learn whether the cloth is stained by the suspect's semen, DNA testing can be done.

Here is how it works:

1. First, it must be shown that the stain contains semen. Semen glows under ultraviolet light and also changes color when exposed to specific chemicals.
2. If these tests are positive, the lab technicians cut a swatch of the fabric from the stain and dissolve the organic matter in the stain by putting the swatch in a special solution.
3. The technicians then look for sperm cells in the solution with an ordinary low-power light microscope. The sperm look like little lifeless tadpoles.
4. If there are visible sperm under the microscope, the techs next extract DNA from the sperm. This is done with a mild detergent that bursts non-sperm cells. A rinse with water removes broken cells. And then a stronger detergent is used to burst the sperm and recover their DNA.
5. The lab then compares the DNA from the semen with the suspect's DNA. A blood or saliva sample from the suspect will supply enough DNA for the comparison.
6. If the stain is small and the amount of DNA minute, a method based on the polymerase chain reaction, or PCR, is employed to make millions of copies of selected segments of the DNA. Note that PCR does not change the DNA but merely amplifies the amount.
7. The DNA from the semen is then compared with the suspect's. This can be done using what are called restriction fragment length polymorphisms, or RFLPs. To examine RFLPs, the DNA from the sample is cut (restricted) by special enzymes. These restriction enzymes cut different DNAs differently. If the restriction enzymes cuts the suspect's sample and the semen sample into the same number of fragments of the same length, then the semen may be the suspect's.
8. The odds that the DNA in the semen is the suspect's are then calculated. The odds may, for example, be one in 11 million that the DNA in the semen came from another man, not the suspect.

The DNA tests of semen can take several days or several weeks before results are available, depending upon which techniques have been used by the lab (and which lab has done the DNA testing). DNA tests can show that the semen is not from the suspect. Or these tests can show that the DNA in the sperm is not detectably different from the suspect's. But DNA testing can never prove with complete certainty that the semen is from the suspect.