 To determine the recommended phase II dose RPTD of GDC-0032 in combination with tamoxifen in hormone receptor positive, HER2 negative metastatic breast cancer patients who have progressed after prior endocrine treatment .Description of toxicity profile, severity and frequency of adverse events observed with the combination of GDC-0032 and tamoxifen To evaluate the safety and tolerability of GDC-0032 in combination with tamoxifen, recording adverse events using CTCAE v. 4.0 criteria To describe the pharmacokinetics of GDC-0032 in combination with tamoxifen To investigate the possibility of major drug-drug interactions PK To obtain proof of target inhibition by selected pharmacodynamic measurements To look for preliminary evidence of anti-tumour activity To assess the status of potential biomarkers for drug response like PIK3CA gene mutations, relevant proteins and phospho-proteins in the PI3K pathway, circulating tumour DNA ct-DNA To assess germline DNA sequence for pharmacogenetics studies 