Initially, 1,721 records were identified through searching five online databases (PubMed, ASCO, San Antonio Breast Cancer Conference, ESMO, and ESMO Breast). After three iterations, 111 records were selected, which fulfilled both the inclusion and exclusion criteria (Fig 1). The molecular biomarkers with the strongest body of evidence for their association with drug response are described below. Molecular Biomarkers for Treatment Resistance Retinoblastoma gene 1.