Among patients with recurrent MBC, those who experienced a change in tumor subtype from primary to first metastatic site (N = 392, 18%) were less likely to have primary lobular histology (7% vs. 13%, p < 0.001), more likely to have a higher primary grade (grade 3: 56% vs. 47%, p < 0.001), less likely to have received CDK4 inhibitors (26% vs. 56%, p < 0.001) and more likely to have received chemotherapy (99% vs. 95%, p < 0.001) in the metastatic setting compared to those who had a consistent subtype from primary to metastatic diagnosis. Here, CDK4 is linked to neoplasm.