The diagnosis of the melanoma-prone disorder dysplastic nevus syndrome (DNS) is based currently on a combination of clinical and histopathologic examinations of patients. To develop a potential laboratory test for DNS, we utilized the observation that an ultraviolet light (UV)-treated mutagenesis plasmid shuttle vector has an abnormally increased frequency of mutations after transfection into lymphoblastoid cells from a patient with familial DNS. pSP189 (containing the bacterial suppressor tRNA gene supF as a marker for mutations and a gene for ampicillin resistance for selection) was treated with UV and transfected into familial DNS, xeroderma pigmentosum complementation group A (XP-A), and normal lymphoblastoid cells by electroporation or diethylaminoethyl (DEAE) dextran. Untreated plasmid pZ189K (containing a gene for kanamycin resistance) was co-transfected as an internal standard to reduce the variability of plasmid survival measurements. After 2 d, plasmids were extracted, used to transform an indicator strain of Escherichia coli, and assayed on plates containing ampicillin or kanamycin. Counting light blue or white colonies (containing mutated supF in the plasmid) and blue colonies (with wild type supF) permitted measurement of the plasmid survival and mutation frequency. Transfection by electroporation or DEAE dextran resulted in abnormally reduced survival of UV-treated plasmid after passage through the XP-A but normal survival in the three DNS lines. Transfection of UV-treated plasmid by DEAE dextran yielded a greater hypermutability with the familial DNS lines than by electroporation. These results suggest that pSP189 UV hypermutability with normal UV survival using DEAE dextran transfection may form the basis of a potential laboratory assay for familial DNS.