In response to questions raised recently about flow cytometry data reported in our paper (Figure 2, Jiang Y, Vaessen B, Lenvik T, Blackstad M, Reyes M, Verfaillie CM. Multipotent progenitor cells can be isolated from post-natal murine bone marrow, muscle and brain. Exp Hematol, 30: 896-904, 2002), we had the flow cytometry data for that figure and the experimental approach used to generate them reviewed by flow cytometry experts. It was their consensus opinion that a number of the plots are flawed, as corresponding IgG isotype control plots for several of the plots differ even though the same IgG subtype was used. Hence the plots in this particular Figure should not be relied upon as accurate representations of MAPC surface marker profiles. While problems with these specific plots undermine their utility as markers of the MAPC surface phenotype, the flawed flow cytometry data do not otherwise alter the conclusions of the paper. Nevertheless, we wish to inform other scientists of the problems with these published FACS profiles. Additional FACS plots subsequently published by my laboratory (Breyers A, Estharabadi N, Ulloa F, et al. Multipotent Adult Progenitor Cell (MAPC) isolation and culture procedures. Exp Hematol. 34: 1596-601, 2006; Serafini M, Dylla SJ, Oki M, Heremans Y, Buckley S, Jiang Y, Bryder D, Rossi DJ, Panoskaltsis-Mortari A, O’Shaughnessy MS, Nelson-Holte M, Weissman IL, Blazar BR, Verfaillie CM. Complete Lymphohematopoietic Reconstitution by Multipotent Adult Progenitor Cells. J Exp Med 204: 129-139, 2007) are not affected by the same flaw in the plots published in our 2002 paper. These profiles provide reliable characterizations of the cell surface phenotype of MAPCs and actually confirm the marker phenotype we originally published in Exp Hematol, 30: 896-904, 2002. Catherine Verfaillie, MD Professor of Medicine Director, Stamcelinstituut, K U Leuven Onderwijs & Navorsing 1 Herestraat 49, bus 804 3000 Leuven