PIK3CA and breast cancer: In particular, PI3KCA (the gene coding for the isoform α of the catalytic subunit of PI3K) is the most common altered gene in this pathway (mutated in ~15% of human cancers and having copy number amplifications in ~5% (Zhang et al., 2017; Zehir et al., 2017)) and is particularly important in the context of breast cancer (mutated in ~35% and copy number amplifications in ~5% (Ciriello et al., 2015; Koboldt et al., 2012; Stephens et al., 2012; Pereira et al., 2016)).