To assess whether Mcl-1 is an essential tumor cell survival factor during a standard of care treatment regimen (i.e. first-line treatment with AIs, followed by second-line treatment with fulvestrant upon AI resistance), we first analyzed matched pairs of clinical breast tumor specimens from patients with ERα+ breast cancers for Mcl-1/Bim interactions by PLA. The gene discussed is ESR1; the disease is breast neoplasm.