ERBB2 and ductal breast carcinoma in situ: Figure 1A illustrates examples of homogeneity and heterogeneity in IHC marker expression of DCIS lesions. Fewer than 10% of the cases exhibited a variable expression pattern for ER and p53, therefore these markers tended to be similar among all DCIS lesions within a single patient (Figure 1B). Conversely, expression of PR, HER2, Ki-67, and p16 were variable in 22, 58, 50 and 61% of cases, respectively, which significantly differed from our hypothesized baseline value of 10% of the cases demonstrating variable expressions of markers (P<0.05 for PR; P<10−8 for HER2, Ki-67 and p16).