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A portion of the observed variation in SCE frequencies among primates might be the result of inherent differences in BrdUrd incorporation. Stetka and Carrano 12 found that the BrdUrd/base pair ratio correlated well with SCE frequency. The incorporation of BrdUrd is not linearly proportional to the molarity of BrdUrd in the culture medium, while the BrdUrd induced SCE frequencies are linearly proportional to the percentage substitution of BrdUrd for 543 dThd 13. It appears that BrdUrd competes with dThd for incorporation into DNA and that this competition determines the frequency of BrdUrd induced SCE. That mutagenicity of BrdUrd is not determined by the amount of the analog incorporated into DNA 14 might not be a valid factor for explaining interspecies differences in BrdUrd induced SCE since increased SCE frequency is not necessarily equivalent to increased mutagenicity 15' i6.Summary. Red blood cell glutathione peroxidase activity was increased by about 50% in all Down's syndrome patients studied. It was slightly lower in translocation as compared with simple trisomy 21 but much higher with respect to controls.

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A statistically significant decrease of oxygen enhancement ratio was found in lymphocytes of Down's syndrome (D.S.) patients, compared with