Objectives: To investigate whether serum alemtuzumab concentrations are predictive of the robustness of engraftment in SCD patients undergoing a matched sibling donor transplantation with alemtuzumab/TBI conditioning resulting in mixed chimerism. Study design: Prospective observational laboratory study. Serum alemtuzumab concentration will be measured at various time points before and after stem cell infusion (days -3, 0, +7, +14, +28, +60). Study population: Adult SCD patients that are planned for a matched sibling donor transplantation with alemtuzumab/TBI conditioning at the Amsterdam UMC. Main study parameters/endpoints: The correlation between serum alemtuzumab concentration and levels of donor chimerism. Secondary endpoints: correlation between serum alemtuzumab levels and patients with and without successful engraftment. Correlation of serum alemtuzumab levels and the dosing of alemtuzumab in mg/kg, number of patient lymphocyte count and total number of infused enucleated donor-derived cells.