Mutations in PIK3CA were more prevalent in claudin-low/ER-positive and/or HER2-positive breast cancers in the METABRIC cohort (42.1%) compared with claudin-low/ER negative/HER2 negative (17.6%, Fisher’s exact test p = 0.0001, Table 4, Figure 1), reflecting the higher prevalence of these mutations in ER-positive disease. Here, ESR1 is linked to breast cancer.