1,25(OH)2D3 has been shown to upregulate IDO resulting in increase of CD4 + CD25+ Tregs in multiple sclerosis (Correale et al., 2009) and 1,25(OH)2D3 induced IDO is a suggested mechanism for downregulation of Th‐1 priming and tolerogenic DC upregulation of Tregs (Gorman et al., 2010). This evidence concerns the gene IDO1 and multiple sclerosis.