To analyze if methotrexate (MTX) resistance arises from gene amplification in a patient treated clinically with MTX, the cytogenetic and drug sensitivity profile of the tumor colony forming units (TCFUs) from a 58-year-old woman with stage III well-differentiated ovarian serous adenocarcinoma was studied. This patient had not received treatment directed against her tumor for nine months before this study, but had received oral-dose MTX (2. 5 mg, twice weekly) for three years for the treatment of psoriasis. Analysis of TCFUs grown in nucleoside-free media demonstrated MTX resistance at concentrations of up to 100 micrograms/mL (2. 2 X 10(-4)M) VSports手机版. Cytologic evidence for dihydrofolate reductase (DHFR) gene amplification in TCFUs was determined by in situ hybridization, using radiolabeled cDNA to DHFR mRNA. Results localized the DHFR sequences to an abnormally staining region present on chromosome 4q. This study supports the notion that alterations in gene dosage (that is, gene amplification) play a role in the development of drug resistance in spontaneous human cancers. .