A total of 80 primary human breast carcinoma DNAs were analysed for loss of heterozygosity (LOH) on the long arm of chromosome 6, using microsatellite markers whose location has been defined physically and by linkage analysis. Loss of heterozygosity was observed in 38 of 80 (48%) tumours that were informative for at least one locus. The analysis revealed partial or interstitial deletions of chromosome 6q. Detailed mapping of chromosome 6q in these tumour DNAs identified two and perhaps three commonly deleted regions. One of these is located between markers D6S251 and D6S252 (6q14-q16.2), another between D6S268 and D6S261 (6q16.3-q23) and a third between D6S287 and D6S270 (6q22.3-q23.1).