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Mutation induction by UV irradiation was studied in a retroviral vector integrated in one copy per cell at various chromosomal positions. As a mutational target, hamster hprt cDNA was present on the retroviral vector. To minimize the influence of repair we used repair-deficient hamster cells, V-H1 and UV5, as a recipient for the vector. There is no major influence of chromosomal position on UV-induced mutation frequency and spectrum because no statistically significant difference between mutation induction in retroviral cDNA copies integrated at different chromosomal sites was observed. However, a major difference was found in mutation induction between the endogenous hamster hprt gene and the retroviral cDNA copies. Most noticeable was the absence in the cDNA of the strong strand bias for mutation induction, which was reported for the endogenous hprt gene. Our results with the hprt cDNA exclude as a general phenomenon a difference in mutation induction for leading and lagging strand DNA replication, which was proposed as an explanation for this strand bias in the endogenous gene. The similarity of mutation induction in the different retroviral cDNA copies, all directly surrounded by the same DNA sequence elements, together with the marked difference between the mutation induction in the endogenous gene and the cDNA copies may point to an important role of chromatin structure in mutation induction.