Genomic analysis of single CTCs reveals important heterogeneity in the mutation of various driver genes (e.g., PIK3CA, ESR1, KRAS, PTCH1, NOTCH1) reflecting the presence of discrete subclonal mutations within the tumour of origin and/or the presence of genomic alterations driving resistance/metastasis [122,123,124,125,126,127]. The gene discussed is ESR1; the disease is neoplasm.