Using our assembly for the capybara (Hydrochoerus hydrochaeris) (a giant rodent), a previous publication20 has identified positive selection on anti-cancer pathways, echoing previous reports21 that other large mammal species—the African and Asian elephants (Loxodonta africana and Elephas maximus indicus, respectively) —carry extra copies (retrogenes) of the tumour-suppressor gene TP53. This offers a possible resolution to Peto’s paradox—the observation that cancer in large mammals is rarer than expected—and could reveal anti-cancer mechanisms. This evidence concerns the gene TP53 and cancer.