Expression levels of PIK3CA, the oncogene bearing the highest number of mutations in breast cancer, were therefore mostly stable in breast cancer subgroups indicating that mutations constituted the main tumor change affecting PIK3CA. These results show that changes of expression of PIK3R1 but not PIK3CA play a role in breast cancer, specifically in hormone receptor-negative cases. The gene discussed is PIK3R1; the disease is breast carcinoma.