The data item Sequence Number refers to the sequence of malignant and non-malignant tumors diagnosed in a patient and is used to distinguish cases with multiple cancer diagnoses. By default, your PUF includes all sequence codes available for each reported patient. Patients with only one lifetime cancer diagnosis will have a sequence number code value of 00. Sequence number 01 indicates that the reported tumor is the first of multiple diagnoses. The NCDB has no mechanism by which to link separate case reports of the same patient. It is customary to limit analyses to patients with sequence numbers 00 and 01 to ensure that any review of treatment or outcomes of the study cohort is not confounded by treatment administered for a prior cancer diagnosis. It is not uncommon to encounter high sequence numbers, especially among melanoma patients.