PTEN and breast cancer: Mutations in PIK3CA and PTEN have been described to be largely mutually exclusive in breast cancers [34]; therefore, we speculated that the detection of co‐occurring PIK3CA and PTEN mutations in 6.3% (n = 32/508) of the baseline ctDNA samples from SANDPIPER participants (Fig. 1B) may indicate these alterations are derived from different cancer cells in a single tumour or from entirely different lesions.