PDCD1 and cancer: Schatton et al. [52] identified tumorigenic human ABCB5+ MMICs (a novel type of cancer stem cells, malignant melanoma-initiating cells), expressing chemoresistance determinant ABCB5 preferentially expressed PD-1 and B7-2, but with downregulated expression of PD-L1 compared to ABCB5− cells [53].