BAF57 is a core subunit present in all mammalian SWI/SNF ATP-dependent chromatin remodeling complexes, which regulates important biological processes including gene transcription, DNA recombination, DNA repair, and DNA replication. Among other functions, BAF57 mediates the recruitment of SWI/SNF to sequence-specific transcription factors. Thus, BAF57 plays a crucial role in regulating estrogen-dependent gene expression and proliferation in human cell lines derived from breast tumors. Increasing genetic and biochemical evidences suggest that mutations in BAF57 or alterations in its expression could play an oncogenic role in the mammary gland. Here, we describe two novel mutations in the BAF57 gene found in a breast cancer patient. Both mutations originate premature stop codons, leading to truncated proteins, structurally similar to another BAF57 mutant previously found in a human cell line derived from a breast tumor (BT-549). The expression of these novel BAF57 mutants has abnormally high estrogen receptor alpha (ERα) coactivating potential, suggesting that they might be involved in the aberrant estrogen-dependent proliferation that occur in the majority of breast tumors that retain ERα expression. In addition, the mutations in BAF57 affect its functional interaction with the androgen receptor and ETS2, two transcription factors that play an important role in breast cell biology. Therefore, mutations in BAF57 could impinge on several oncogenic signaling pathways contributing to the origin and/or development of breast cancer.