ESR1 and Pleural effusion: We used: (1) luminal MCF7 cells (WT TP53 and ER+), which depend on estrogen and EGF (Epidermal Growth Factor) for their growth and are neither locally invasive nor metastatic in mouse models, (2) the triple-negative MDA-MB-231 (mutant TP53-R280K, ER-), which was derived from a pleural effusion metastasis, and (3) its ‘D3H2LN’ variant (mutant TP53-R280K, ER-), which was selected for their enhanced tumor growth and widespread metastasis in mice (Jenkins et al., 2005).