Large-scale massively parallel sequencing studies have revealed that breast cancers display a complex repertoire of somatic mutations, that TP53 (37%), PIK3CA (36%), and GATA3 (11%) are the only three genes recurrently mutated in >10% of unselected breast cancers, and that the repertoire of somatic mutations differs between ER-positive and ER-negative disease; however, no pathognomonic mutations underpinning ER-positive or ER-negative breast cancers have been identified6,7. Here, PIK3CA is linked to breast cancer.