Due to RPS6KB1 amplification and overexpression in breast cancer and the role of p70S6K as a downstream mediator of PI3K/mTOR pathway, our aim was to identify PI3K/mTOR/p70S6K pathway downstream targets using gene expression profiling for breast cancer cell lines that we have previously characterized in regard to copy number and gene expression [4,19]. The gene discussed is PIK3CA; the disease is breast carcinoma.