Although combination regimens such as anti-PD1 with anti-cytotoxic T-lymphocyte antigen 4 typically show higher response rates,7 their benefit in XP remains unclear – particularly given the apparent heightened sensitivity to monotherapy seen in our case and in previous reports.8–10 In this context, the incremental efficacy of combined therapy is uncertain, whereas the increased risk of toxicity and higher cost is well established. The gene discussed is PDCD1; the disease is xeroderma pigmentosum.