1. Department of Pediatric Oncology, Hematology and Clinical Immunology, Center for Child and Adolescent Health, Heinrich Heine University, Düsseldorf, Germany;
2. Institute for Transplantation Diagnostics and Cell Therapeutics, Heinrich Heine University, Düsseldorf, Germany; and
3. Department of Pathology and Centre for Trophoblast Research, University of Cambridge, Cambridge, United Kingdom