Touch DNA is predominantly transferred from leukocytes, not skin cells

DNA extracted from touched surfaces is an important analyte in forensic casework, often leading to the identification of suspects. A common assumption underlying this approach is that touch DNA originates in skin cells of the touching individual. Here we test this assumption, using cell type-specific DNA methylation markers to determine the origins of touch DNA. Strikingly, hand swabs and touched objects contain only traces of skin DNA. The majority of hand swab and touch DNA originates in leukocytes (mostly neutrophils) and to a lesser extent in buccal cells, paralleling the composition of nasal and salivary DNA. Hand swab DNA from females that received bone marrow transplantation from a male donor are predominantly of a male origin, consistent with blood rather than skin origin. Finally, the amount of DNA extracted from hand swabs does not increase after prolonged glove use, further arguing against internal shedding. We conclude that touch DNA is transferred mostly from the nose or mouth, not from the skin of the touching hand. The leukocytic origins of touch DNA have implications for understanding DNA transfer and for the interpretation of forensic findings.