ESR1 and breast cancer: Interestingly, during the 48 h treatment time the dual HER2/EGFR inhibitor lapatinib, currently in use for second-line therapy of herceptin-resistant ER+EGFR+HER2+ breast cancers, did not reduce the pLB subpopulation, consistent with the fact that HER2– tumors (like pLB) tend to be unresponsive to this drug [19].