PIK3CA and breast cancer: Copy number (CN) alterations in ctDNA were also associated with breast cancer subtype (Fig. 1d and Supplementary Fig. 2), with CN alterations in MYC, PIK3CA, EGFR, CCNE1, CDK6, BRAF and MET more common in TNBC (q = 0.01, q < 0.0001, q = 0.001, q < 0.0001, q = 0.0003, q = 0.0002 and q = 0.01, respectively, Fig. 1d).