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The presence of the Rhesus (Rh) complex on human red cells is important in establishing for blood type compatibility, but the exact biochemical role of these proteins has eluded discovery for over six decades. In 28 May online Proceedings of National Academy of Sciences USA, Eric Soupene and colleagues from University of California at Berkeley, show that expression of the RH gene is regulated by CO₂ and suggest that Rh may be a CO₂ channel (Proc Natl Acad Sci USA 2002, 99:7769-7773).

Soupene et al. examined RH1 gene expression in the green alga Chlamydomonas reinhardtii — a photosynthetic microbe. They observed that expression of RH1 was high in cells grown in air supplemented with 3% CO₂ or shifted from air to high CO₂ (3%) for 3 h. Conversely, RH1 expression was low for cells grown in air (0.035% CO₂) or shifted from high CO₂ to air for 3 h.

Given ...