Background: Currently, one of the most urgent problems in breast cancer therapeutics is the validation of targeted therapy for triple-negative breast cancer (TNBC). Neratinib is an irreversible small molecule inhibitor that targets the kinase activity of EGFR, HER2 and HER4. As one of these targets, EGFR, is expressed at high levels in a subset of TNBC, we hypothesised that neratinib is a potential treatment for at least some patients with TNBC. Methods: The antiproliferative effects of neratinib were investigated in a panel of 35 breast cancer cells lines including 9 luminal, 12 HER2-positive and 14 triple-negative (TN). Baseline levels of total HER proteins were determined using flow cytometry. Results: IC₅₀ values for neratinib across the panel of breast cancer cell lines ranged from < 0.001 - 1.9 µM. HER2-positive cell lines were significantly more sensitive to neratinib than HER2-negative cells (p = 0.008, Student’s t-test). For the 14 TN cell lines, IC₅₀ values ranged from 0.03 – 1.9 µM. TN cell lines defined as basal-type A were significantly more sensitive to neratinib than those designated basal-type B (p = 0.02, Student’s t-test). Treatment with neratinib resulted in a reduction in HER-family signalling as detected by decreased Akt and ERK phosphorylation. Consistent with the finding that basal type A TN cells were more sensitive to neratinib than type B, significantly higher levels of EGFR expression were found in the A subtype compared to the B subtype (p = 0.02, Student’s t-test). To determine a possible predictive marker of response for neratinib, levels of all 4 HER proteins were related to response in the basal A subgroup. Baseline levels of HER3 expression were found to correlate significantly with response to neratinib (p = 0.02, r = -0.875, n = 7, Spearman), i.e., cell lines with the highest levels of HER3 expression had the lowest IC₅₀values within this cohort. Conclusions: Our results suggest that neratinib is a potential novel therapeutic option for patients with basal A type triple negative breast cancer. HER3 may act as a predictive marker of response for this subgroup of patients. Acknowledgements: The authors thank SFI for funding MTCI (SRC award, 08/SRC/B1410 to MTCI) and an SFI Short Term Travel Fellowship to MM, for funding this work.