The invention features methods of diagnosing high grade ductal carcinoma insitu (DCIS). These methods involve measuring: (1) the level of HID-5 in a bodyfluid (e.g., blood or urine) of a subject suspected of having, or at risk ofhaving, high grade DCIS； or (2) the level of HID-5 gene expression in breasttissue from a subject suspected of having, or at risk of having, high gradeDCIS. The invention also embodies a method of inhibiting expression of HID-5protein in DCIS cells and methods of treating a subject suspected of having,or at risk of having, high grade DCIS.
