L. Vigilant et al. (1) recently presented "the strongest support yet for the placement of [their] common mtDNA [mitochondrial DNA] ancestor in Africa some 200,000 years ago." This support stems from a tree estimated by maximum parsimony from mtDNA sequence data with the use of the computer program PAUP (2). The African origin is inferred from this tree because (i) the most basal splits are amon...

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In the treatment of superficial lesions with 8-13 MeV electrons, lead shields are often used to protect the underlying tissue. Measurements were made with film and ion chamber to analyse various aspects of external and internal shielding in electron beam therapy. Data were obtained on the thickness of lead required for shielding, the effect of blocking on dose-rate, electron-backscattering from...