PDCD1 and cancer: The low to undetectable expression of PDCD1 in cancer cell lines may be attributable to its expression only in a subpopulation of cancer cells.21 22 Therefore, single-cell RNA-sequencing of cancer cells might be helpful in validating the expression of PDCD1 in specific cancer cell subpopulations and characterizing the signaling involved in the regulation of PDCD1 in cancer cells.