 B2: Patients n20 with PIK3CA mutant advanced solid tumours including at least 8 patients with PIK3CA mutant ER negative andor HER2 positive breast cancers will be treated with the doublet combination of palbociclib and either taselisib or pictilisib. Other cancers with relevant genetic aberrations e.g. KRAS mutations may be considered, depending on emerging preclinical and clinical data on these novel antitumour agents. 