Across the CTCs and ctDNA, common regions of loss included chromosome 13q14 containing the RB1 gene, detected in 3 of 4 patients receiving CDK4/6 inhibitors and amplification of 17q12 containing the ERBB2 gene in 2 of the 7 patients with HER2 negative metastatic disease, suggesting evolution to HER2 positive disease. The gene discussed is RB1; the disease is metastatic neoplasm.