To study the involvement of DNA mismatch-repair genes in sporadic breast cancer, matched normal and tumoral DNA samples of 22 patients were analysed for genetic instability and loss of heterozygosity (LOH) with 42 microsatellites at or linked to hMLH1 (3p21), hMSH2 (2p16), hMSH3 (5q11–q13), hMSH6 (2p16), hPMS1 (2q32) and hPMS2 (7p22) loci. Here, PMS1 is linked to breast carcinoma.