This proof-of-concept trial is aiming at evaluating CALCIneurin activity as a monitoring biomarker of efficacy of cyCLOsporine - (CALCICLO) - for the prophylaxis of acute GVHD. Our aim is to assess whether a longitudinal monitoring of CN activity would permit to adapt and optimize the dose of CsA that would prevent the onset of severe acute GVHD, yet still maintaining an acceptable tolerability profile.