A Western blot and quantification of the levels of HER2, EGFR, and beta-actin in breast cancer cell lines, newly established cell lines, their corresponding PDX-derived organoids and PDX tumors. Expression of HER2 and EGFR for MC-BR-BTY-0019 cell line, organoid, and PDX was normalized to beta-actin and then SKBR3. Expression of HER2 and EGFR for MC-BR-BTY-0006 cell line, organoid, and PDX was normalized to beta-actin and then MDA-MB-231. All blots are derived from the same experiment and were processed in parallel. B MC-BR-BTY-0019 and MC-BR-BTY-0006 cell lines were stained with HER2 and DAPI and images were taken at ×100. Scale bar represents 10 μm. C MC-BR-BTY-0019 and MC-BR-BTY-0006 cell lines were stained with EGFR and DAPI and images were taken at ×100. Scale bar represents 10 μm. D 10,000 cells/well of MC-BR-BTY-0019 cell line were seeded in 96-well plates in triplicates, treated with indicated concentrations of lapatinib for 48 h, and cell viability was measured by MTS assay and normalized to DMSO. IC50 = 438.531 nM. E 10,000 cells/well of MC-BR-BTY-0006 cell line were seeded in 96-well plates in triplicates, treated with paclitaxel at indicated concentrations for 48 h, and cell viability was measured by MTS assay and normalized to DMSO. IC50 = Indeterminate. F, G PDX tumors were collected when the tumor achieved 10–20 mm in diameter, and primary breast cancer cells were isolated after dissociation of tumor tissue from mouse cells, as previously described7. MC-BR-BTY-0019 PDX-derived organoids were generated by seeding 10,000 cells of the primary breast cancer single-cell suspension per well in Nanoculture plates in triplicates and treated with indicated concentrations of lapatinib for 5 days (IC50 = 979.49 nM). MC-BR-BTY-0006 PDX-derived organoids were treated with indicated concentrations of paclitaxel for 5 days (IC50 = Indeterminate). Organoid viability was measured using 3D Cell TiterGlo viability assay, normalized to DMSO, and plotted using GraphPad PRISM software. Error bars represent SEM.