To assess the functionality of DDN in mediating sensitivity to inhibition of PI3K by GDC-0941, and to further probe the observed linear relationship between basal DDN gene expression (Fig. 2C) and de novo sensitivity to GDC-0941, we used siRNA pools targeting DDN to knockdown the expression of DDN in the GDC-0941-resistant MDA-MB-231 breast tumor cell line, and in a second breast tumor cell line, the GDC-0941-sensitive MCF-7 breast tumor cell line. This evidence concerns the gene PIK3CA and breast neoplasm.