Figure 1. Classification of human CD4⁺FOXP3⁺ T cells. In humans, CD4⁺FOXP3⁺ T cells can be classified into three subsets: naïve Tregs (Fr.1), eTregs (Fr.2), and non-Tregs (Fr.3). These three fractions can be distinguished based on the expression of CD45RA, cell surface markers of naive T cells, and the transcription factor FOXP3. Moreover, these subpopulations are functionally different in terms of their suppressive activity. Effector Tregs harbor strong immune suppressive activity, but non-Tregs do not possess immune suppressive activity. In the majority of cancer, eTregs predominantly infiltrate into tumor tissues. In general, the frequency of eTregs in cancer patients is 2~5% in peripheral blood but approximately 10~50% in the tumor tissues. In contrast, naïve Tregs and FOXP3⁺ non-Tregs are insufficient or absent altogether.