Whereas an IHC-based profiling of DCIS did not associate ALDH1 with breast cancer events (9), our MS-based analysis on paired DCIS/IDC lesions does show a significantly higher concentration of ALDH1 in DCIS compared with IDC lesions (P = 0.01), supporting findings from stem cell biology that ALDH1 might be a functional and prognostic biomarker of tumorigenesis in DCIS. This evidence concerns the gene ALDH1A1 and breast cancer.