As well as helping to identify suspects at the scene of crimes and in the courtroom, DNA testing is also spreading as a useful tool to help exonerate those falsely accused of crimes. While tests are hailed as extremely accurate, they can also become a double-edged sword depending on how they are used.
"Even if it's just one day earlier, we would like his retrial to begin and for him to be found innocent," says the family of 81-year-old Iwao Hakamada. In 1966, a family of four in Shizuoka Prefecture was killed in what came to be known as the "Hakamada Incident." Hakamada was sentenced to death over a "confession" obtained during interrogation that he later retracted during trial. However, in 2014, a DNA test opened up a path to a retrial: the blood on clothes found near the crime scene thought to belong to the murderer did not match Hakamada's blood. He has since been released to await a new trial, even though prosecutors have appealed the decision.
Recently, DNA tests have been used as evidence to prove innocence in some cases to overturn indefinite "life" sentences, such as that of the Ashikaga murder case in 1990 and the 1997 murder of a female Tokyo Electric Power Co. employee.
In April 2016, "Innocence Project Japan" was founded by lawyers and DNA testing experts to support those claiming to have been wrongfully convicted and requesting a retrial. When undertaking the process of getting the case taken back to court, cooperation from both scientific and legal experts is essential. The inspiration for the project came from a similarly named NPO "Innocence Project" in the United States, which has already cleared the names of over 300 wrongly-convicted people using the latest DNA testing methods. Innocence Project Japan hopes to be the Japanese arm of the movement.
However, in some cases, the DNA left at a crime scene may not be preserved well and is impossible to test. In response, a method that analyzes the short tandem repeats of a DNA sequence was developed to efficiently test even limited amounts of DNA. Fragmented DNA sequences made up of only 200 to 400 nucleobases can be tested using this method. Polymerase chain reaction (PCR), which replicates a given DNA chain, is also used. The method uses heat and its namesake DNA polymerase, enzymes that make DNA bases, to first separate the double-stranded DNA molecule into two strands before filling in the matching nucleobase pairs. Using PCR, the amount of DNA in a sample can reportedly be increased by hundreds of thousands of times.
If DNA testing is carried out under appropriate conditions, it has such a high accuracy that it can identify a single person out of more than 100 quintillion people.
However, as DNA testing is such a powerful tool that presents compelling evidence identifying a person, its use is subject to the whims of the humans involved in a case, sometimes being used and other times not during the course of the investigation or trial. That is where the problem with the testing lies.
In a 2012 rape case tried in the Kagoshima District Court, the Kagoshima Prefectural Police collected DNA evidence, but the sample was deemed too small to be accurately tested. In the man's first trial, he was found guilty via other evidence. However, during his appeal trial, the DNA sample was run and it ended up not being a match. It became clear that the prefectural police had discarded the remaining DNA sample and the results of the test. This led to suspicion that the police had deliberately hid the DNA evidence in order to gain a guilty verdict. The man's sentence was overturned in 2016.
"Generally, it's taboo to put too much faith in DNA tests," says Toho University professor of forensic medicine Kunihiko Kurosaki. There may be other substances mixed into the sample, or a mishandling of the testing procedure could result in inaccurate results.
As for Hakamada, the prosecution has raised questions about a method of a DNA test that the defense counsel employed, which found that he was not a match, and immediately filed an appeal.