ESR1 and neoplasm: A mutation in ESR1 (D538G), which has been shown to impart resistance to estrogen deprivation therapy, was found in both biopsies of the metastases and the cfDNA.[30, 31] This mutation was not present in the initial exome sequence of the primary tumor and its absence was confirmed by subsequent validation sequencing of ESR1 to a depth of 4,272X (Fig 6A).