We provide genetic evidence supporting the identity of the candidate gene for BRCA 1 through the characteri-zation of gerrnline mutations in 63 breast cancer patients and 1 0 ovarian cancer patients in ten families with cancer linked to chromosome 17q21. Nine different mutations were detected by screening BRCA 1 DNA and RNA by single-strand conformation polymorphism analysis and direct sequencing. Seven mutations lead to protein truncations at sites throughout the gene. One missense mutation (which occurred independently in two families) leads to loss of a cysteine in the zinc binding domain. An intronic single basepair substitution destroys an acceptor site and activates a cryptic splice site, leading to a 59 basepair insertion and chain termination. The four families with both breast and ovarian cancer had chain termination mutations in the N-terrninal half of the protein.In October 1994, a candidate gene for BRCAl, which is responsible for inherited predisposition to breast and ovarian cancers in some families, was isolated bypositional cloning 1 โข In this report, we confirm that BRCAl is this predisposing gene by analysing germline mutations in the families that originally defined the linked phenot}'pe2, as well as in other families with breast cancer, and often ovarian cancer, linked to chromosome 17q21.For this purpose, the BRCAl gene was screened using single strand conformation polymorphism ( SSCP) analysis in 20 BRCAl-linked families, using both genomic DNA and eDNA prepared from lymphoblast RNA. Criteria for distinguishing cancer-predisposing mutations from polymorphisms in BRCAl were (i) cosegregation of the variant with breast cancer, and with ovarian cancer, if it appeared; (ii) absence of the variant in control chromosomes; and (iii) amino acid substitution in, or truncation of, the BRCA1 protein encoded by the variant sequence.