The HER-2/neu (erbB-2) oncogene, if amplified and/or overexpressed in breast and ovarian cancers, is associated with a poor prognosis. Employing direct DNA sequencing, we have discovered and sequenced an 80 base pair intron from human placenta which contains an A to G polymorphism. This polymorphism lends itself to restriction fragment length polymorphism analysis of the PCR product spanning this intron. All three genotypes, homozygous A, heterozygous, and homozygous G appear in normal control populations and breast tumors. Also, no difference was seen between the polymorphic form found in five breast cancers and the corresponding normal tissue.