PIK3CA and breast cancer: A significant enrichment in genes mutated in luminal B breast cancers was observed in the constellation of mutations found in MPCs; out of the 119 genes most frequently mutated (ie ≥ 4 cases – 3.5%) in luminal B IC-NSTs from The Cancer Genome Atlas (TCGA) study 16, TP53 (W53*), PIK3CA (H1047R), CSMD2 (R3608S), MAP3K1 (insertions and deletions in three cases), ATRX (P667T), HMCN1 (A537G), MLL2 (A946E), SPEN (ESS2280A), and ZFHX4 (A2896S) were mutated in MPC samples (representation factor = 2.3; hypergeometric test p value < 0.01; Table2).