Of the 98 ER+ and ER- breast cancers from women with BRCA1 mutations, the status of the wt BRCA1 allele could not be obtained for 21 cancers (12 ER- and 9 ER+): in 4 cancers (1 ER- and 3 ER+), microdissection for isolation of a pure population of tumor cells was not feasible due to the intimate admixture of single tumor cells and normal cells; in 10 cancers (8 ER- and 2 ER+) PCR failed; in 7 cancers (3 ER- and 4 ER+) the results from triplicate PCR were too variable (that is, CV > 20%) for inclusion in analysis. The gene discussed is ESR1; the disease is breast cancer.