MUC5AC and invasive ductal breast carcinoma: As we all know, ductal adenocarcinoma is an invasive mucin‐producing and duct‐forming epithelial neoplasm with an intense stromal desmoplastic response, often presents low neoplastic cellularity, only 5%‐20%.26 Prior genome sequencing studies employed techniques to purify tumor samples, which makes that samples with low neoplastic cellularity have been underrepresented, even though low cellularity cancers comprise the majority of surgically resected PDAC.27 Besides, there are only four normal control cases in TCGA‐PAAD.