Relevant to their high diabetes susceptibility, the acquisition of Idd5.1 (68) by F2 ΔtTreg mice correlated with increased expression levels of ICOS (Icos) and PD-1 (Pdcd1) on pTreg cells (Figures 9A, C), and the accumulation of an unusual ICOS+PD-1high pTreg cell subset in F2 ΔtTreg mice, but not in Foxp3RFP/GFP littermate controls (Figure 9D). Here, ICOS is linked to diabetes mellitus.