Concurrent ESR1/ERBB2 mutation was more frequent in ctDNA than tissue: in ctDNA, 4% (3/79) of ESR1-mutated cases had concurrent ERBB2 mutation; whereas, in the FM database, 0.6% (6/969) of ESR1-mutated BC tissue samples had concurrent ERBB2 mutation. The gene discussed is ERBB2; the disease is breast cancer.