 Part B1: At the MTD dose expansion, fulvestrant will be administered in addition to palbociclib and taselisib or pictilisib orally once daily, in a 21-days-on and 7-days-off schedule in the ERve HER2-ve PIK3CA mutant breast cancer cohort. Fulvestrant will be given intramuscularly on Day 1, Day 15 in cycle one followed by Day 1 for all subsequent cycles. 