Another study conducted in a relative large cohort of MBC patients, using droplet polymerase chain reaction (dPCR) on circulating tumor DNA (ctDNA), showed that ESR1 mutations in patients, with MBC receiving prior AI treatment, was observed in 18/128 cases (14%), with D538G mutations but 21% of patients carry ESR1 polyclonal mutations. This evidence concerns the gene ESR1 and neoplasm.