Soon after reproductive and developmental biologist Sudhansu Dey's group at the University of Kansas moved to new quarters at Vanderbilt University in Nashville, Tenn., they began noticing that once well-established results on uterine gene expression and reproductive function in female mice procured from the same supplier, and of the same genetic strain, had become a lot less predictable. Three years later, they seem to have discovered the ghost in the cage: rodent chow.

In a study published last month in the Proceedings of the National Academy of Science (102:9960–5, 2005), Dey and his colleagues describe feeding female mice two types of Purina Mills LabDiet, a leading brand of rodent chow, containing markedly different amounts of isoflavones. This family of molecules is common in food plants such as soy and alfalfa and can simulate estrogen. Mice at Vanderbilt typically eat the high-isoflavone Purina mix, while those at Kansas were fed another...

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