We have constructed a set of hybrid cell lines by irradiation of GM 64063 (human chromosome 9q only on hamster background) and fusion to hamster A23 Tk-, 109 independent lines were tested by PCR with 24 markers from chromosome 9q. The marker density is highest in the 9q22.3-q31 region containing the gene for Gorlin syndrome, a familial predisposition to basal cell carcinoma. The resolution of our map in this region is significantly higher than other published maps and will enable accurate placing of new markers and genes within the 9q22.3-q3.1 region. This is important since yeast artificial chromosomes from the region are likely to contain deletions.