Aims: Trophoblast fusion in the placenta is prerequisite to successful pregnancy and the pathological conditions related to it. The presence of syncytin-1, is not sufficient to explain the complete event and ADAM12 is a major co-player candidate. Via differential splicing, the ADAM12 gene produces a short and a long form, being the ADAM12-S and the ADAM12-L respectively. Methods and results: We investigated the localisation of both variants in the human placenta using whole mount in situ hybridisation, immunohistochemistry and Northern blotting in 1st (n = 8) and 3rd (n = 8) trimester placentae and in the case of NB in several cell lines. In Northern blotting, 1st and 3rd trimester placentae were positive for the ADAM12-S and Bewo, 293HEK, JAR, leucocytes, macrophages, 1st and 3rd trimester placentae were positive for ADAM12-L. In whole mount in situ hybridisation, the 1st and 3rd trimester placental syncytium was positive for both variants. In immunohistochemistry, ADAM12-L localised in the cytotrophoblast of both 1st and 3rd trimester placentae, while ADAM12-S localised in the complete syncytium, often including the cytotrophoblast. Conclusion: The different localisation of ADAM12-S and ADAM12-L indicates a possible different role making ADAM12-L a candidate for the fusion event, while the syncytial localisation of the ADAM12-S makes it a candidate for cell-cell and cell-matrix interactions between the placental syncytium and the maternal interface. (C) 2011 Elsevier GmbH. All rights reserved.