A human breast cancer cDNA library was screened with a polyclonal antiserum against breast cancer metastasis membranes in order to select genes whose expression may be altered in breast malignancy. Several immunopositive clones have been isolated. One of these, AJ1, was analyzed in detail and found to be expressed at varying levels as a 3.3 kb mRNA in all of the 123 primary breast cancers examined. AJ1 was expressed at lower levels in non-malignant biopsies as compared to malignant tissue (p=0.0008). We found high expression to be associated with lymph node involvement (p=0.059). Comparison between high and low expressing groups showed a significant difference for both overall (p=0.039) and relapse-free (p=0.034) survival at 10 years, with early recurrence and reduced overall survival in high expressors. Thus, AJ1 could be a potentially useful prognostic factor and marker of aggressiveness in breast cancer. Partial sequence analysis of the 620 bp clone showed complete homology with human heat shock protein, HSP 90.