PIK3CA and maternal uniparental disomy of chromosome 20: In a review of 14 series of MBCs [13,15,16,17,18,26,27,28,30,31,32,42,44,114], with a total of 539 tumors, we observed that the genes most frequently mutated were TP53 and several genes of the PI3K pathway, such as PIK3CA, and of the WNT pathway, such as APC (see Figure 3).