PIK3CA and breast cancer: In lapatinib-resistant HER2-positive breast cancer cells, PI3K/AKT and MAPK signaling was maintained despite sustained suppression of the HER2 tyrosine kinase, and the tyrosine phosphorylated proteomes of both sensitive and drug-resistant cells were analyzed by immunoenrichment mass spectrometry, which revealed that the phosphorylation of Src family kinases and presumptive Src substrates in some resistant cell lines was increased.