ARID1A and breast cancer: The extent of molecular disparity became more pronounced when comparing individual molecular subtypes where enrichment of PTEN mutations was identified in YBC TNBC tumors, whereas OBC tumors showed chromosomal amplification of SRSF2. Additionally, YBC HR + tumors harbored loss-of-function mutations in DNA damage response genes, including ARID1A and PMS2. In the case of HER2 + tumors, we discovered that OBC patients generally exhibited increased levels of PIK3CA mutations, suggesting potential clinical implications of PIK3CA-mediated therapy for HER2 + BC patients.