PDCD1 and melanoma: Although Taggart et al., using a mouse model of brain metastasis established by intracranial injection of melanoma cells, reported that the presence of subcutaneously transplanted extracranial tumors and the activation/release of CD8+ T cells were required for the anti-PD-1 and anti-CTLA-4 combination to exert its therapeutic effect on intracranial tumors [45], in our model we did not visually detect any extracranial lesions after the intra-carotid artery injection of MBT2 cells.