QCCs were identified in all DCIS samples (22/22) at a mean percentage (QCC-P) of 0.95% ± 0.93 per patient sample (one section), similar to the frequency in invasive breast cancer (Fig. 2a).7,8 Given the small number of samples, it was not possible to test an association between ER or HER2 receptor status and QCC-P. Here, ESR1 is linked to ductal breast carcinoma in situ.