Interestingly, a very recent study by Matson et al. examining the stool samples collected from patients with metastatic melanoma before anti-PD-1 immunotherapy found that B. longum, Collinsella aerofaciens, and Enterococcus faecium were more abundant in the anti-PD-1 immunotherapy responders, supporting the anti-tumor effects of Bifidobacterium species (71). Here, PDCD1 is linked to metastatic melanoma.