Sharks are not cancer-proof. Authoritative medical sources and peer-reviewed studies document both benign and malignant tumors in sharks and explicitly identify the claim as a myth. Research on low mutation rates or distinctive immune genes may suggest biological differences, but it does not show that sharks never develop cancer.
Caveats
The claim uses absolute wording; documented cancer cases in sharks are enough to disprove it.
Evidence about low mutation rates, cartilage, or cancer-related genes does not logically imply zero cancer incidence.
Cancer in wild sharks may be underdetected, but uncertainty about prevalence is not evidence that sharks do not get cancer.