Immunohistochemical expression of ISG15 in normal breast tissue, and in non-invasive and invasive breast tumours using a tissue microarray. (a, b) Normal breast tissue lacks ISG15 expression (IRS = 0). (c, d) In ductal carcinoma in situ ISG15 expression is more intense (IRS = 8) than in normal breast tissue. (e, f) In invasive breast carcinomas (here: ductal type) ISG15 expression was clearly more abundant (example shows staining with an IRS = 12) compared with normal breast tissue and ductal carcinoma in situ. (g, h) In tubular breast carcinomas (IRS = 4), a less frequent variant of invasive breast carcinomas with a more favourable prognosis than invasive ductal breast carcinomas, ISG15 expression was less abundant than in most invasive ductal breast carcinomas and stronger than in most normal breast tissues. Magnifications: a, c, e, g: 100×; b, d, f, h: 400×.