A novel cDNA clone was Isolated using a polyclonal serum directed against partially purified ovarian carcinoma antigen CA125. The deduced peptide sequence lacked membrane protein characteristics expected for CA125 but encompassed a B-box/coiled coll motif present in many genes with transformation potential. The gene was mapped by fluorescence In situ hybridization within the minimum region known to contain the familial breast/ovarian carcinoma gene, BRCA1. YAC and cosmid clones were isolated and used to refine the location of this gene adjacent and proximal to the RNU2 locus. The exon structure of the gene was determined. Extensive SSCP and sequence analysis of over 100 tumour and normal DNAs from familial and sporadic breast cancers and sporadic ovarian cancers failed to detect mutations In the coding region of this gene.