Although there was no difference between the ratios of Treg subsets and the FOXP3 protein level in cKO mice compared with WT mice (Figure S10G,H, Supporting Information), the inhibitory function of Treg cells was downregulated in cKO mice compared with WT mice (Figure 4E and Figure S10I (Supporting Information)), which is consistent with a previous study showing that SOCS1 is essential for maintaining Treg cell function.[25] These data suggested that Socs1 deficiency in T cells activated T cells and might induce severe GVHD in an HSCT mouse model. Here, SOCS1 is linked to graft versus host disease.