We analyzed 16 Italian breast and breast/ovarian cancer families for BRCA1 germline mutations using a combination of the protein truncation test (PTT) and the single-strand conformation polymorphism techniques. Genomic DNA from the affected proband of each family was analyzed by applying the PTT to exon 11 of the BRCA1 gene. This initial screening led to the identification of truncated protein products in three families that were shown to carry three different frameshift mutations. In the families that scored negative in the PTT, single-strand conformation polymorphism analysis of the entire coding sequence of the gene revealed four additional mutations consisting of one nonsense, one in-frame deletion, one frameshift, and one missense mutation (in a family with a case of male breast cancer). The four frameshift mutations resulted in a decreased expression of the mutant allele, whereas no loss of transcript was associated with the other three mutations. All mutant alleles were shown to cosegregate with the cancer phenotype within the families, and none have previously been reported.