Given that HTLV-1 carriers—the population at risk for ATLL—are present not only in endemic regions (e.g., southwestern Japan, the Caribbean, parts of South America) but also among immigrant populations in Europe and North America, clinicians must vigilantly assess HTLV-1 serostatus and consider regional epidemiology before initiating PD-1 inhibitor therapy. Here, PDCD1 is linked to adult T-cell leukemia/lymphoma.