A recent cohort study exploring the potential association between blood serum 25 OH vitamin D(25[OH]D) levels and Th17 and Treg, and Treg/ICOS+ levels in healthy siblings of children with T1D reveals that Treg/ICOS+ percentages are higher in siblings with lower 25(OH)D levels and higher genetic risk for T1D. This evidence concerns the gene ICOS and type 1 diabetes mellitus.