Taken together, our findings demonstrate a high degree of variability in Ruxolitinib concentrations and significantly higher drug exposure in GvHD patients in comparison to patients with MF receiving the same dose, most likely caused by frequent comedication with several moderate and weak CYP3A4 and CYP2C9 inhibitors, often in combination with azoles (strong CYP3A4 inhibitors). This evidence concerns the gene CYP3A4 and graft versus host disease.