CD4 and type 1 diabetes mellitus: As regards T1D, a decreased expression of PD-1 gene was observed in CD4+ T cells of patients with autoimmune T1D and in recent studies [46,47], CD4+ T cells of Japanese T1D patients carrying the 7785 C/C genotype of the PDCD1 gene showed lower PD-1 expression than those with the C/T and T/T genotypes.