Emerging resistance-conferring genetic alterations found using ctDNA include mutations in ESR1 (codes for ER) in patients with breast cancer who develop resistance to aromatase inhibitors [81,82], reversion mutations in BRCA1/2 genes in patients with diverse solid cancers that develop resistance to PARP inhibitors [83] and mutations in FGFR2 in patients with cholangiocarcinoma who become resistant to FGFR inhibitor [84]. The gene discussed is ESR1; the disease is cholangiocarcinoma.