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The activity of superoxide dismutases (SOD) I and 2 was analysed in correlation with mRNA and chromosome content in 6 SV40‐transformed (TF) and in non‐transformed (NF) human fibroblast cell lines. Total SOD activity was fairly constant, whereas the ratio SOD2/SOD I was much lower in TF than in NF. The decrease in SOD2 activity was correlated with a low mRNA content, and with the presence of various chromosomal rearrangements leading to deletions of the long arm of chromosome 6 where the gene is mapped. In contrast, chromosome 21, carrying the gene for SODI, was not found to be deficient and the SODI activity was high. This shows that in TF, the activity of SOD2 is largely determined by gene dosage. It has been proposed that SOD activity could be inversely correlated with cell proliferation, and that SOD2 activity, in particular, was related to cell differentiation. Thus, there is a cascade of events …