Dickkopf-1 expression in breast cancer samples and serum. (A) Dickkopf-1 (DKK-1) expression was assessed in a breast cancer tissue microarray consisting of 74 breast cancer samples and 10 normal tissues. The TMA was stained with an antibody directed against human DKK-1 and staining was graded from none (0) to strong (3). (B) A cDNA breast cancer array was used to quantify DKK-1 expression in breast cancer. Expression of DKK-1 was measured and normalized to β-actin. (C) DKK-1 serum levels are elevated (P = 0.006) in patients with estrogen receptor (ER)-negative breast cancer (n = 28) or in healthy controls (n = 27). Patients’ characteristics are listed in Figure S3B in Additional file 1.