Taken together, we observed that the most frequently mutated genes in ER+/PR− breast cancers are PIK3CA, TP53, GATA3, CHD1, KMT2C, MUC16, MAP3K1, ARID1A, AHNAK2, and SYNE2. Interestingly, PIK3CA and TP53 show a mutational prevalence (37% and 33%, respectively) that differs significantly to that of ER+/PR+ tumors (with PIK3CA mutated in 47% of cases and TP53 in 19%). Here, ARID1A is linked to breast cancer.