Cutaneous squamous cell carcinoma (cSCC) is the second-most common cancer in United States, and its incidence is substantially higher in men than women, but the reasons for the difference are unknown. Researchers explored whether common mitochondrial DNA (mtDNA) haplogroups, which have been associated with cancer risk, and in particular squamous cell carcinoma risk arising in other organs, could explain this biological sex difference in cSCC susceptibility.