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The majority (approximately 75%) of infant acute leukaemias have a reciprocal translocation between chromosome 11q23 and one of several partner chromosomes. The gene at 11q23 (named MLL, ALL-1, HRX or HTRX-1; refs 2-6) has been cloned and shares homology with the Drosophila developmental gene trithorax. Rearrangements of this gene (called HRX here) occur in introns and cluster in a region of approximately 10 kb; individual patients have different breakpoints. Here we describe three pairs of infant twins with concordant leukaemia who each share unique (clonal) but non-constitutive HRX rearrangements in their leukaemic cells, providing evidence that the leukaemogenic event originates in utero and unequivocal support for the intra-placental 'metastasis' hypothesis for leukaemia concordance in twins.

All seven of a set of CD34 monoclonal antibodies that recognize epitopes on an approximately 110 Kd glycoprotein on human hemopoietic progenitor cells also bind to vascular endothelium. Capillaries of most tissues are CD34 positive, as are umbilical artery and, to a lesser extent, vein, but the endothelium of most large vessels and the endothelium of placental sinuses are not. Angioblastoma cells and parafollicular mesenchymal cells in fetal skin are also CD34 positive, as are some stromal elements. An approximately 110 Kd protein can be identified by Western blot analysis with CD34 antibodies in detergent extracts of freshly isolated umbilical vessel endothelial cells, and CD34 mRNA is present in cultured umbilical vein cells as well as other tissues rich in vascular endothelium (breast, placenta). These data indicate that the binding of CD34 antibodies to vascular endothelium is to the CD34 gene product, and not to crossreactive epitopes. Despite the presence of CD34 mRNA in cultured, proliferating endothelial cells, the latter do not bind CD34 antibodies. In addition, CD34 antigen cannot be upregulated by growth factors. We conclude that under these conditions, CD34 protein is downregulated or processed into another form that is unreactive with CD34 antibodies. Electron microscopy of umbilical artery, breast, and kidney capillary vessels reveals that in all three sites, CD34 molecules are concentrated on membrane processes, many of which interdigitate between adjacent endothelial cells. However, well-established endothelial cell contacts with tight junctions are CD34 negative. CD34 may function as an adhesion molecule on both endothelial cells and hematopoietic progenitors.