Differential extraction refers to the process by which the DNA from two different types of cells can be extracted without mixing their contents. The most common application of this method is the extraction of DNA from vaginal epithelial cells and sperm cells from sexual assault cases in order to determine the DNA profiles of the victim and the perpetrator. Its success is based on the fact that sperm cells have protein disulfide bonds in their outer membrane which makes them more resilient to extraction than epithelial cells.

After determining that sperm cells are present (typically through staining and light microscopy) in a vaginal/rectal sample, the subject’s epithelial cells are lysed and their DNA extracted through normal means. The epithelial DNA in solution is removed and saved, while the sperm cells are still intact. Differential extraction uses a chemical called dithiothreitol (DTT) to disrupt the sulfur bonds in the coating of the sperm cell in order to extract its DNA. Once the sperm’s outer membrane has been breached, it is prone to standard DNA extraction methods. This creates two different DNA fractions from one sample, hopefully that of the victim and that of the perpetrator.