To assess PI3K signaling activity in the panel of breast cancer cells used for the present investigation, the levels of phosphorylated forms of AKT, S6 protein kinase 1 and S6 (indicators of PI3K signaling activation), and the expression of PI3K catalytic subunit isoforms, PTEN, AKT isoforms and mTOR were examined (Figure 1). Here, PIK3CB is linked to breast cancer.