In this study, a novel somatic point mutation that changed a cytosine nucleotide into a thymine nucleotide at position of 2830 from the start codon of SIN3A (c.2830 C>T) was detected in a breast cancer tissue showing high expression levels of ESR1. The mutation is a nonsense mutation that changes a codon for glutamine into a stop codon (p.Gln944*) at position of 944 from that start methionine. Here, SIN3A is linked to breast cancer.