Hormonal history is a major risk factor in the development of cancers of the breast and ovary. The effects of the steroid hormones estrogen and progesterone on growth and development are mediated through cytoplasmic receptors encoded by the genes ESR and PGR, previously mapped to 6q24-q27 and 11q22-q23, respectively. To localize ESR more precisely the authors isolated a YAC containing the gene and mapped it by fluorescence in situ hybridization (FISH) and a new, simple method of post-FISH chromosome banding. Comparisons of the fluorescent signals and subsequent G-banding of a total of 39 chromosomes were performed independently by two qualified cytogenetists by aligning the center point of each fluorescent signal with the corresponding G-banded picture. This allowed the mapping of the YAC and hence ESR to 6q25.1