Reply to: HEP-21-0529.R1.

Thank you for your letter and the attention to important risk associations between smoking and immunosuppression with malignancy. As you rightly mention, the limitations of the UNOS database preclude analyzing those factors in our study and others like it. Unfortunately, these types of large databases do not provide information on many possible cofounding factors including smoking status, prior surgical history including a prior colectomy, family history, and specifics regarding a patient's history of malignancy prior to transplant (exceptions including hepatocellular carcinoma and cholangiocarcinoma).