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.