In my experience, there can be considerable variation in DNA synthesis in control groups of hairless mice. This occurs even though the animals are kept in a controlled environment with standard food and light-dark conditions. Also, my experiments were performed during a period of several months. I have found that a vial of tritiated thymidine can lose its specific activity even though refrigerated, and this could be a factor causing a variation in counts from experiment to experiment over time. It was for these reasons that dorsal skin was compared with ventral skin (paired-data design) to try to minimize factors that cause variations in DNA synthesis between animals. There do not appear to be significant differences in DNA synthesis when dorsal and ventral skin are compared in the same animals.1The mice were not restrained to prevent all movement so that it was possible to get some
Walter J. Sunscreens-Reply. Arch Dermatol. 1982;118(12):960. doi:10.1001/archderm.1982.01650240003003
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