Summary Investigation of genetic changes in tumours by loss of heterozygosity (LOH) is a powerful technique for identifying chromosomal regions that may contain tumour suppressor genes. LOH has been described on chromosome 6 in ovarian carcinoma using restriction fragment length polymorphism analysis with a small number of probes. We studied 29 ovarian carcinomas with 19 probes mapping to chromosome 6. Sixteen of the 29 tumours showed LOH on 6q (55%). Of these 16, 63% showed loss of all informative markers on that arm. One tumour showed loss of 6q24-qter, localising the putative tumour suppressor gene to that region. Loss on 6p was 28% overall. However, using three dinucleotide repeat primer pairs from 6p to study LOH in seven (Friend et al., 1986), with the finding that a germline mutation constituted the first 'hit', to be followed by a second, somatic inactivation of the gene, which was usually detectable by RFLP analysis of filters of DNA from matched normal-tumour pairs. More recently, it has been shown that some families with retinoblastoma share a common mutation that can be traced from affected parent to affected child. In these cases, the other allele is lost or inactivated in various ways that differ in different affected family members (Phillips et al., 1991).For soine years there has been considerable cytogenetic evidence that in OC, one chromosome 6, particularly the long arm, is missing in part or in whole (Mitelman, 1991 seven had breakpoints between 6q21 -23, thus loss of the long arm telomeric to this region was the commonest single abnormality of chromosome 6 in this study. Despite the common occurrence of aneuploidy, Atkin et al. (1983) showed that one copy of 6q may be lost in early stage tumours, when the karotype is relatively undisturbed. These data have been followed up more recently by molecular studies of LOH that have broadly confirmed the cytogenetic findings (Ehlen & Dubeau, 1990;Lee et al., 1990).Further data are now needed to accurately define regions of LOH, so that efforts can be concentrated on cloning genes in the relevant region of chromosome 6. Therefore we have studied 29 pairs of matched malignant tumour and normal DNA with ten DNA markers that detect polymorphic sequences on 6q and six that do so on 6p. We also used the centromeric marker p308 (D6ZJ). Oka et al. (1991) showed that by using carefully dissected tumours it was possible to reliably demonstrate LOH by PCR. Subsequently dinucleotide (microsatellite) repeats mapping to chromosome 17 were used to successfully demonstrate LOH in breast cancer (Futreal et al., 1992). Therefore we included the dinucleotide repeats D6S89 and FTHPI, which were utilised to study the regions 6p22.3-23 and 6pl2-21 respectively by PCR-LOH. From the use of these eight 6p probes in all the tumours, we selected seven tumours for a more extensive analysis with three further dinucleotide repeats mapping to the region 6pl2-6p23. We also included 12 nonmalignant ovarian tissue specimens in our RFLP analysis. Although not all these samples wer...