It is well known that 1,25 (OH)2D3 (vitamin D) can modulate immune cells, for instance, we found that vitamin D as well as a CatG inhibitor impair the activation of proinsulin-reactive T cells (auto-reactive T cells) from T1D patients, because CatG is elevated in peripheral blood mononuclear cells from T1D patients and enhances insulin antigen processing (Zou et al., 2011). Here, CTSG is linked to type 1 diabetes mellitus.