PTEN and breast cancer: To investigate how PTEN loss or deficiency are common in ER+ BC biopsies harbouring PIK3CA alterations impacted sensitivity to PI3K-AKT pathway inhibitors we identified two ER+ breast tumour PDX models with activating mutations in PI3KCA and alterations in PTEN. The first with a homozygous deletion of PTEN (ST3932) resulting in reduced protein expression, and the second with a hemizygous deletion of PTEN (CTG3283) resulting in intermediate PTEN protein expression compared to models with no alterations in PTEN (labelled PTENwt) (Fig. 3A)23–27.