In 1996 LEP ran at a centre-of-mass energy of 161 GeV, just above the threshold of W-pair production. DELPHI accumulated data corresponding to an integrated luminosity o f 9 : 93 pb 1 , and observed 29 events that are considered as candidates for W-pair production. From these, a cross-section for the dou-

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65P early in their life cycle (phase GI), and cells near the bottom at the end of the cycle (phase G2). The experiment with labelled thymidine shows that cells near the middle of the gradient are in the period of DNA synthesis (phase S). These results, in confirmation of other work (Bergeron, Warmsley & Pasternak, 1969), indicate that incorporation of proline, uridine and choline into protein, RNA and phospholipid respectively is not limited to a specific portion of the cell cycle (as is the case with thymidine), nor is the rate of incorporation constant throughout the cell cycle. Rather incorporation appears to increase from a low rate in phase Gl to a higher rate in phase G2. Such an increase has been observed by using other techniques in the case of RNA and protein synthesis (e.g.

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