PDCD1 and neoplasm: When we investigated Tregs lacking Helios expression (FoxP3+Helios−), we found a strong positive correlation between levels of these Tregs and CD4+PD-1+ T cells (r = 0.627, p = 0.002) in tumor tissues, while moderate correlations were found between levels of these cell subsets in normal tissues (r = 0.443, p = 0.039) but not in PBMCs (Figure 4A).