The germlines of male monozygotic (MZ) twins: Very similar, but not identical

•Male MZ twins may carry somatic mutations that allow discrimination of their germlines by whole genome sequencing (WGS).•The approach, which started off as a thought experiment, has since been applied to real-life cases by our laboratory.•In line with theoretical expectation, the search for MZ discriminating mutations was successful in four of six cases.•Further case work is required for WGS-based discrimination of male MZ twin germlines to become generally acceptable in court. In 2012, a thought experiment in this journal suggested that paternity cases involving monozygotic (MZ) twins as putative fathers could be solved by means of whole genome sequencing (WGS). Although arising from a single fertilization event, MZ twins nevertheless continue to acquire somatic mutations during their development, including those that occur in the germline. Provided that paternity had been narrowed down to the twin pair beforehand by classical DNA analysis, one post-zygotic mutation would suffice to assign the paternal compartment of an offspring genome unambiguously to either twin if that mutation is found in the offspring and one twin, but not in the other twin. Since the publication of a proof-of-principle report in 2014, we have worked up five additional cases of MZ twin germline discrimination in real life, four paternity disputes and one criminal case requiring the identification of a sperm trace donor among a pair of MZ twin brothers. In this opinion paper, we report on the experiences made in the course of our work and take a look at possibilities for further development of the approach.