Interestingly, we found elevated levels of ARG1-positive myeloid cells predominantly inside the metastatic tumour, consistent with previous reports showing similar results in 4T1 [36] and PyMT-BO1 [40] metastatic breast tumours as well as colon carcinoma, lymphoma and melanoma [37] and glioma [41] mouse models. This evidence concerns the gene ARG1 and central nervous system cancer.