ESR1 and neoplasm: A plausible cause for the low uptake of [18F]FES in this study is the relatively low expression of ER in the SKOV3 tumor cell line (ca. 830 fmol/mg of protein) compared with, for example, the frequently used ER-expressing MCF7 breast tumor cell line (ca. 1900 fmol/mg of protein) [18, 19].