Police who are investigating a series of sexual assaults in the southern French city of Marseille have arrested identical twin brothers.
The 24-year-old unemployed delivery drivers, named locally as Elwin and Yohan, were placed under investigation on Friday.
Officers say they are sure that one of the two men carried out the attacks, but that they do not know which.
Standard DNA tests are unable to differentiate between their DNA.
I wonder what is the ultra-sophisticated genetic test they are referring to; presumably no genetic test can be any more sophisticated than a whole genome sequence at very high coverage, and that does not cost a million euros anymore.
Police have been told it would cost upwards of 1m euros (£850,000) to conduct an ultra-sophisticated genetic test that would be able to tell one set of the twins' DNA from the other.
One expert told the French newspaper La Provence: "For a normal analysis, we would compare 400 base pairs [of nucleotides] which make up DNA."
In the case of identical twins, he added, "We would be looking at billions."
The closest I could find on the topic of twin genetic similarity is the following reporting on an ASHG 2012 abstract:
They then calculated the frequency with which these mutations occurred. Only two sets of twins had such mutations, which translates to a DNA change occurring once for every 10 million to 10 billion bases that are copied every time a cell divides. While that may seem like a high accuracy rate, cells in the body divide trillions of times. So that would mean an average twin pair carries 359 genetic differences that occurred early in development.
One limitation of the study is that they could only estimate the mutation rate based on blood cells, but some cells in the body divide much more frequently and so may rack up many more mutations. Other cells, like brain cells, don't regenerate much and would probably remain stable.