A new DNA test will help police in the UK solve rape cases even when the attacker has not left any traces of sperm, scientists say.
Semen may not contain any sperm
The test looks for the genetic hallmark of other cells present in semen, such as skin and immune cells.
Its developers at Birmingham's Forensic Science Service say the test will help provide DNA evidence in a further 90 sexual assault cases a year.
It was used in a case for the first time in January, New Scientist reports.
A suspect was charged and the case is now being heard at court.
A common problem for forensic scientists hoping to use DNA fingerprinting to identify assailants in sexual assault cases is that the quantity of male DNA in swabs taken from the woman is often tiny compared with the amount of hers present.
Conventional methods of amplifying small amounts of DNA do not work because the female DNA would be amplified as well.
Andy Hopwood and colleagues at the Forensic Science Service (FSS) looked at ways to isolate the male component.
One way is to use enzymes that destroy the coating of ordinary cells, leaving intact only sperm cells.
But this is not possible in every case - in 10-15% of the cases the FSS deals with, no sperm can be found in the semen, either because of a medical condition or because the male has had a vasectomy.
Instead, the FSS scientists have combined a technique called laser microdissection (LMD), which enables any male cells carrying the Y chromosome to be extracted from a microscope slide, with fluorescence in-situ hybridisation (FISH), which can highlight unique DNA sequences within cells.
With these, and the FSS's super-sensitive DNA Low Copy Number analysis, the scientists were able to obtain accurate male genetic fingerprints from vaginal swabs taken up to 24 hours after sexual intercourse, even when no sperm were present.
FSS research manager Martin Bill said: "It will help us to obtain vital DNA profiles in a further 90 sexual assault cases a year that have previously been just too difficult to get a usable DNA profile from."
Professor James Fraser, director of the centre for forensic science at the University of Strathclyde, said: "This is a breakthrough because it has the potential to provide evidence in a range of cases that hitherto would not be found."