SLC6A8 and neoplasm: Further study of the tumor-infiltrating CD8 T cell subsets revealed an upregulation of CrT gene expression that was much more significant in the PD-1hi cells compared to the PD-1lo cells; the highest levels of CrT gene expression were detected in the PD-1hiTim-3hiLAG-3hi subset that is considered to be the most “exhausted” [44].