Interestingly, in contrast to experimental models of brain metastasis derived from lung cancer or breast cancer (Fig. 2b, Supplementary Fig. 2f,g, Extended Data Figs. 3a and 4d and Supplementary Fig. 4a), we were unable to find a melanoma brain metastasis model expressing S100A9 (Supplementary Fig. 5a), potentially suggesting that a specific subtype of melanoma brain metastases, not represented by available mouse models, could be the main contributor to S100A9 positivity. Here, S100A9 is linked to breast cancer.