Degenerate expression of transcription coregulator proteins is observed in most human cancers. Therefore, in targeted anti-cancer therapy development, intervention at the level of cancer-specific transcription is of high interest. The steroid receptor coactivator-1 (SRC-1) is highly expressed in breast, endometrial, and prostate cancer. It is present in various transcription complexes, including those containing nuclear hormone receptors. We examined the effects of a peptide that contains the LXXLL-motif of the human SRC-1 nuclear receptor box 1 linked to the cell-penetrating transportan 10 (TP10), hereafter referred to as TP10-SRC1(LXXLL), on proliferation and estrogen-mediated transcription of breast cancer cells in vitro. Our data show that TP10-SRC1(LXXLL) induced dose-dependent cell death of breast cancer cells, and that this effect was not affected by estrogen receptor (ER) status. Surprisingly TP10-SRC1(LXXLL) severely reduced the viability and proliferation of hormone-unresponsive breast cancer MDA-MB-231 cells. In addition, the regulation of the endogenous ER alpha direct target gene pS2 was not affected by TP10-SRC1(LXXLL) in estrogen-stimulated MCF-7 cells. Dermal fibroblasts were similarly affected by treatment with higher concentrations of TP10-SRC1(LXXLL) and this effect was significantly delayed. These results suggest that the TP10-SRC1(LXXLL) peptide may be an effective drug candidate in the treatment of cancers with minimal therapeutic options, for example ER-negative tumors.