Pre-clinical findings show that ROCK2 inhibition reduces production of Th17 signature cytokines IL-17 and IL-21 in a STAT3-dependent manner, while increasing STAT5 signaling, thereby promoting Treg function (19, 20), have led to clinical studies testing ROCK2 inhibitors in GVHD, which we discuss later in this chapter. This evidence concerns the gene IL21 and graft versus host disease.