Study on reproducibility evaluation of STR typing of Touch DNA

Short Tandem Repeat (STR) typing plays an important role in forensic science. Currently, many laboratories return the residual unextracted DNA evidence sample to police stations according to regulations in Japan. However, it has not been clarified how well the results of the original DNA analysis can be reproduced in the case of retest of trace or mixed DNA from remaining unextracted DNA evidence samples. In this study, simulated touch DNA samples were prepared, and STR typing was performed to verify the reproducibility of the results. Touch DNA samples were produced by having 25 individuals hold a doorknob, a 50 mL tube, and another personʼs clothing tightly for 1 minute each. These touch DNA samples were divided into two parts and STR typing was performed on each and the results were compared. The results indicated that the number of alleles and peak heights may vary between the original DNA test and retest, and the results are not necessarily identical. The discordance in the results may be due to changes in the amount of DNA extracted from the material, changes in the mixture ratio, or the appearance of alleles of unknown origin. This is due to the heterogeneity of touch DNA samples collected by swabbing, etc., and is not considered to negate the reliability of the respective result of DNA tests. The reproducibility of results when re-extracting from residual touch DNA evidence sample was expected to be difficult due to trace or mixed DNA, but to our knowledge, there are no previous reports verifying this fact. This study reveals for the first time the reality of this situation.