MKI67 and breast cancer: Second, while the SEER database contains approximately 28% population-based cancer registration data, some significant confounding prognostic factors including but not limited to Ki-67 index (39), BRCA1- and BRCA2-related mutation (40, 41), as well as high 21-Gene Recurrence Score (21-GRS) (42), which have been proved to be related to worse survival in patients with breast cancer, are unavailable in the SEER database.