AbstractThe ADAMs (a disintegrin and metalloprotease) are membrane proteins containing both protease and adhesion domains and thus may be potentially important in cancer invasion and metastasis. The aim of our study was to investigate the distribution and potential clinical significance of ADAM‐9 in breast cancer. ADAM‐9 expression was measured using both reverse transcriptase‐polymerase chain reaction (RT‐PCR) and Western blotting. ADAM‐9 mRNA was expressed more frequently in both breast carcinomas (72/110, 66%) and fibroadenomas (21/38, 55%) compared to normal breast tissue (6/25, 24%) (p = 0.0004, p = 0.028, respectively). Multiple forms of ADAM‐9 protein were detected by Western blotting, i.e., at 124, 84 and 48 kDa under reducing conditions and at 115, 76, 55, 52 and 46 kDa under nonreducing conditions. The 84 and 55 kDa forms were detected more frequently in the primary cancers compared to normal breast tissue (p < 0.0001, p = 0.0002, respectively). In addition, relative levels of the 84 kDa mature form were significantly higher in the primary cancers than in the fibroadenomas (p = 0.003), while the reverse was found for the 124 kDa precursor form (p = 0.026). In the carcinomas, the 84 kDa form of ADAM‐9 protein was expressed at higher levels in node‐positive than node‐negative cancers (p = 0.05) and correlated positively with HER‐2/neu protein levels (r = 0.313, p = 0.016). This is the first report to describe expression of any ADAM in a large number of human carcinomas. © 2003 Wiley‐Liss, Inc.