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I We previously identified a polymorphism in the human estrogen receptor (ER) gene, within the coding 'I region for the protein's amino terminal B-domain. In estrogen receptor-positive (ER+) breast tumors, the variant allele was preferentially associated with lower levels of ER, and was clinically correlated with i frequent spontaneous abortions. DNA sequencing revealed a point mutation that changes codon 86 from Ala to Val and a silent mutation in codon 87. Because we initially detected the variant allele by analyzing RNA, only those tissues in which the ER gene is actively expressed were suitable for genotype analysis. We now I describe an assay that uses genomic DNA as the substrate for determining the ER B genotype; DNA containing the polymorphic region of the ER gene is amplified by the polymerase chain reaction, then the t amplified DNA is hybridized with radiolabeled oligonucleotide probes complementary to the wild type and variant ER alleles. This method allowed us to determine the ER B genotype of women with ER+ and ER- 1 tumors, starting with minute amounts of DNA from frozen or paraffin embedded tissues. ER B genotyping was also performed on women without breast cancer using DNA extracted from blood cells. The combined results from analyses of RNA and DNA from 300 breast cancer patients showed that 12 % were hetero-zygotes. In the ER+ group (n = 183), 11.5 % carried the variant gene compared to 12.8 % in the ER-negativ