Erythroid. The presence of a split chimerism between mature erythrocytes and their progenitor has been recently described in persistent mixed chimera patients (M. Andreani, Haematologica, 2010, in press). In these patients, there was a predominant erythroid engraftment in the peripheral blood while the proportion of ery- throid precursors (BFU-E [burst forming unit erythroid] colonies) and nucleated cells of donor origin in the bone marrow were equivalent. This observation of split chimerism between mature erythrocytes and their progenitors confirms that a limited HSC engraftment (up to 20%) bearing effective erythropoiesis is able to provide a sufficient amount of normal hemoglobin. Moreover, this limited engraftment with normal erythropoiesis is able to inhibit, through unknown mechanisms, the expansion of the ineffective thalassemic erythropoiesis, as well as the well-known associated clinical consequences.